#include <bits/stdc++.h>
using namespace std;
int n, k;
char ch[111111];
int main() {
  scanf("%d%d", &n, &k);
  scanf("%s", ch);
  int pos = 0;
  for (int act = 0; act <= 1000000 && act < k && pos < n - 1; ++act) {
    if (act >= 1000000 && act % 2 == k % 2) break;
    while (pos < n - 1 && !(ch[pos] == '4' && ch[pos + 1] == '7')) pos++;
    if (pos == n - 1) break;
    if (pos % 2 == 0)
      ch[pos] = ch[pos + 1] = '4';
    else
      ch[pos] = ch[pos + 1] = '7';
    --pos;
    if (pos < 0) pos = 0;
  }
  printf("%s\n", ch);
}
