SNSS 2012 Round 3 Problem E

import java.awt.Point;
import java.io.*;
import java.math.BigInteger;
import java.util.*;

import static java.lang.Math.*;

public class E implements Runnable{

    BufferedReader in;
    PrintWriter out;
    StringTokenizer tok = new StringTokenizer("");

    boolean DEBUG = new File("input.txt").exists();

    void init() throws FileNotFoundException{
        if (!DEBUG){
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(System.out);
        }else{
            in = new BufferedReader(new FileReader("input.txt"));
            out = new PrintWriter("output.txt");
        }
    }

    String readString() throws IOException{
        while(!tok.hasMoreTokens()){
            try{
                tok = new StringTokenizer(in.readLine());
            }catch (Exception e){
                return null;
            }
        }
        return tok.nextToken();
    }

    int readInt() throws IOException{
        return Integer.parseInt(readString());
    }

    long readLong() throws IOException{
        return Long.parseLong(readString());
    }

    double readDouble() throws IOException{
        return Double.parseDouble(readString());
    }

    public static void main(String[] args){
        new Thread(null, new E(), "", 256 * (1L << 20)).start();
    }

    long timeBegin, timeEnd;

    void time(){
        timeEnd = System.currentTimeMillis();
        System.err.println("Time = " + (timeEnd - timeBegin));
    }

    long memoryTotal, memoryFree;


    void memory(){
        memoryFree = Runtime.getRuntime().freeMemory();
        System.err.println("Memory = " + ((memoryTotal - memoryFree) >> 10) + " KB");
    }

    void debug(Object... objects){
        if (DEBUG){
            for (Object o: objects){
                System.err.println(o.toString());
            }
        }
    }

    public void run(){
        try{
            timeBegin = System.currentTimeMillis();
            memoryTotal = Runtime.getRuntime().freeMemory();
            init();
            solve();
            out.close();
            time();
            memory();
        }catch (Exception e){
            e.printStackTrace(System.err);
            System.exit(-1);
        }
    }

    class Domino{

        int first, second;

        public Domino(int first, int second) {
            this.first = first;
            this.second = second;
        }

        int getOther(int value){
            if (value == first){
                return second;
            }

            if (value == second){
                return first;
            }

            return -1;
        }

        boolean equals(Domino d){
            return (first == d.first && second == d.second
                    ||
                    first == d.second && second == d.first);

        }
    }

    int MAX_VALUE = 8;

    void solve() throws IOException{
        int n = readInt();

        Domino[] d = new Domino[n];
        for (int i = 0; i < n; ++i){
            d[i] = new Domino(readInt(), readInt());
        }

        int lim = (1 << n);
        long[][][] dp = new long[MAX_VALUE][MAX_VALUE][lim];

    a:  for (int i = 0; i < n; ++i){
            for (int before = 0; before < i; ++before){
                if (d[before].equals(d[i])){
                    continue a;
                }
            }

            dp[d[i].first][d[i].second][(1 << i)] = 1;

            dp[d[i].second][d[i].first][(1 << i)] = 1;
        }

        for (int first = 1; first < MAX_VALUE; ++first){
            for (int mask = 1; mask < lim; ++mask){
                for (int last = 1; last < MAX_VALUE; ++last){
                it: for (int cur = 0; cur < n; ++cur){
                        if (check(mask, cur)){
                            int next = d[cur].getOther(last);
                            if (next == -1){
                                continue;
                            }

                            for (int before = 0; before < cur; ++before){
                                if (check(mask, before) && d[cur].equals(d[before])){
                                    continue it;
                                }
                            }

                            int newMask = (mask | (1 << cur));
                            dp[first][next][newMask] += dp[first][last][mask];
                        }
                    }
                }
            }
        }

        long ans = 0;
        for (int i = 1; i < MAX_VALUE; ++i){
            ans += dp[i][i][lim-1];
        }

        out.print(ans);
    }

    boolean check(int mask, int index){
        return (mask & (1 << index)) == 0;
    }
}