#include <bits/stdc++.h>
using namespace std;
int main() {
  ifstream ff("file.txt");
  int n, k;
  cin >> n >> k;
  string s1;
  cin >> s1;
  n = s1.length();
  if (k == 0) {
    cout << s1 << endl;
    return 0;
  }
  int j = 0;
  bool b1 = false, b2 = false;
  for (int i = 0; i < n - 2; i++) {
    j++;
    if ((i + 1) % 2 != 0 && s1.substr(i, 3) == "447") {
      b1 = true;
      break;
    } else if ((i + 1) % 2 != 0 && s1.substr(i, 3) == "477") {
      b2 = true;
      break;
    }
  }
  if (b1 == false && b2 == false) {
    j++;
  }
  if (b2 == true) {
    j--;
  }
  for (int i = 0; i < j; i++) {
    if (s1.substr(i, 2) == "47") {
      if ((i + 1) % 2 == 1) {
        s1[i] = '4';
        s1[i + 1] = '4';
      } else {
        s1[i] = '7';
        s1[i + 1] = '7';
      }
      k--;
    }
    if (k == 0) {
      cout << s1 << endl;
      return 0;
    }
  }
  if (b1 == true) {
    if (k % 2 != 0) {
      if (j % 2 == 0) {
        s1[j] = '4';
      } else {
        s1[j] = '7';
      }
    }
  }
  if (b2 == true) {
    if (k % 2 != 0) {
      if (j % 2 == 0) {
        s1[j + 1] = '4';
      } else {
        s1[j + 1] = '7';
      }
    }
  }
  cout << s1 << endl;
  return 0;
}
