#include <bits/stdc++.h>
using namespace std;
string s;
int find_47(const string& s, int from) {
  for (int i = from; i < (s.size() - 1); i++) {
    if ((s[i] == '4') && (s[i + 1] == '7')) return i;
  }
  return -1;
}
int main() {
  int n, k;
  cin >> n >> k;
  cin >> s;
  int from = 0;
  while (true) {
    if (k == 0) break;
    int ind = find_47(s, from);
    if (ind == -1) break;
    if (ind % 2 == 0) {
      if (ind + 2 >= n) {
        s[ind + 1] = '4';
        break;
      }
      if (s[ind + 2] == '7') {
        if (k % 2 == 1) s[ind + 1] = '4';
        break;
      } else {
        s[ind + 1] = '4';
        k--;
        from = ind + 2;
      }
    } else {
      if (s[ind - 1] == '4') {
        if (k % 2 == 1) s[ind] = '7';
        break;
      } else {
        s[ind] = '7';
        k--;
        from = ind + 1;
      }
    }
  }
  cout << s << endl;
  return 0;
}
