import java.awt.Point;
import java.io.*;
import java.math.BigInteger;
import java.security.KeyStore.Builder;
import java.util.*;

import static java.lang.Math.*;

public class Testt {

    final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
    BufferedReader in;
    PrintWriter out;
    StringTokenizer tok = new StringTokenizer("");

    void init() throws FileNotFoundException {
        if (ONLINE_JUDGE) {
            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()) {
            tok = new StringTokenizer(in.readLine());
        }
        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 Testt().run();
    }

    public void run() {
        try {
            long t1 = System.currentTimeMillis();
            init();
            solve();
            out.close();
            long t2 = System.currentTimeMillis();
            System.err.println("Time = " + (t2 - t1));
        } catch (Exception e) {
            e.printStackTrace(System.err);
            System.exit(-1);
        }
    }

    boolean f(long x) {
        while (x > 0) {
            long y = x % 10;
            if (y != 4 && y != 7) {
                return false;
            }
            x /= 10;
        }
        return true;
    }

    // solve! good luck :)
    public void solve() throws IOException {
        int n = readInt();
        int k = readInt();
        n++;
        String s = readString();
        char[] c = new char[s.length() + 1];
        for (int i = 1; i < n; i++) {
            c[i] = s.charAt(i - 1);
        }
        String t = "444";
        String l = "477";
        s = "";
        a: while (true) {
            for (int i = 1; i < n; i++) {
                s += c[i];
                if (s.equals("47") || s.equals("447") || s.equals("477")) {
                    if (s.equals("47")) {
                        if ((i - 1) % 2 == 0) {
                            c[i - 1] = '7';
                            k--;
                        } else {
                            c[i] = '4';
                            k--;
                        }
                    }
                    if (s.equals("477")) {
                        if ((i - 2) % 2 == 0) {
                            k--;
                        } else {
                            if (k % 2 != 0) {
                                c[i - 1] = '4';
                                break a;
                            } else {
                                if (k % 2 == 0) {
                                    break a;
                                }
                            }
                        }
                    }
                    if (s.equals("447")) {
                        if ((i - 2) % 2 == 0) {
                            k--;
                        } else {
                            if (k % 2 != 0) {
                                c[i - 1] = '7';
                                break a;
                            } else {
                                if (k % 2 == 0) {
                                    break a;
                                }
                            }
                        }

                    }
                }
                if (!s.equals("4") ||  (!s.equals("47")) || (!s.equals("44"))){ s="";};
            }
            break a;
        }
        for (int i = 1; i < n; i++) {
            out.print(c[i]);
        }
    }
}