package Difficulty8;

import java.io.IOException;
import java.io.InputStream;
import java.util.*;
import java.util.function.Function;

public class programmingteamselection {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder(1000);
        InputReader in = new InputReader(System.in);
        while (true) {
            int pairs = in.nextInt();
            if (pairs == 0) break;
            boolean[][] graph = new boolean[15][15];
            Map<String, Integer> mapping = new HashMap<>();
            for (int i = 0; i < pairs; i++) {
                Function<String, Integer> f = new Function<>() {
                    @Override
                    public Integer apply(String s) {
                        return mapping.size();
                    }
                };
                int a = mapping.computeIfAbsent(in.readString(), f), b = mapping.computeIfAbsent(in.readString(), f);
                graph[a][b] = true;
                graph[b][a] = true;
            }
            boolean[] used = new boolean[mapping.size()];
            if (used.length % 3 != 0) {
                sb.append("impossible\n");
                continue;
            }
            sb.append(canDo(graph, used) ? "possible\n" : "impossible\n");
        }
        System.out.print(sb.toString());
    }

    private static boolean canDo(boolean[][] graph, boolean[] used) {
        int start = 0;
        while (start < used.length && used[start]) start++;
        if (start == used.length) return true;
        used[start] = true;
        for (int i = start+1; i < used.length; i++) {
            if (used[i] || !graph[start][i]) continue;
            used[i] = true;
            for (int j = i+1; j < used.length; j++) {
                if (used[j] || !graph[start][j] || !graph[i][j]) continue;
                used[j] = true;
                if (canDo(graph, used)) return true;
                used[j] = false;
            }
            used[i] = false;
        }
        return false;
    }

    static class InputReader {
        private final InputStream stream;
        private final byte[] buf = new byte[8192];
        private int curChar, snumChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int snext() {
            if (snumChars == -1)
                throw new InputMismatchException();
            if (curChar >= snumChars) {
                curChar = 0;
                try {
                    snumChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (snumChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = snext();
            while (isSpaceChar(c)) {
                c = snext();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = snext();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = snext();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public long nextLong() {
            int c = snext();
            while (isSpaceChar(c)) {
                c = snext();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = snext();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = snext();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextInt();
            }
            return a;
        }

        public double nextDouble() {
            int c = snext();
            while (isSpaceChar(c)) {
                c = snext();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = snext();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') {
                if (c == 'e' || c == 'E') {
                    return res * Math.pow(10, nextInt());
                }
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = snext();
            }
            if (c == '.') {
                c = snext();
                double m = 1;
                while (!isSpaceChar(c)) {
                    if (c == 'e' || c == 'E') {
                        return res * Math.pow(10, nextInt());
                    }
                    if (c < '0' || c > '9') {
                        throw new InputMismatchException();
                    }
                    m /= 10;
                    res += (c - '0') * m;
                    c = snext();
                }
            }
            return res * sgn;
        }

        public String readString() {
            int c = snext();
            while (isSpaceChar(c)) {
                c = snext();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = snext();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public String nextLine() {
            int c = snext();
            while (isSpaceChar(c))
                c = snext();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = snext();
            } while (!isEndOfLine(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        private boolean isEndOfLine(int c) {
            return c == '\n' || c == '\r' || c == -1;
        }

        public interface SpaceCharFilter {
            boolean isSpaceChar(int ch);
        }
    }
}