#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
using namespace std;
long long mulmod(long long a, long long b, long long MOD) {
  if (b == 0) return 0;
  long long res = mulmod(a, b >> 1, MOD);
  res += res;
  res %= MOD;
  return (b & 1) ? (a + res) % MOD : res;
}
int gcd(int a, int b) {
  while (a != 0 && b != 0) {
    if (a > b) {
      a %= b;
    } else {
      b %= a;
    }
  }
  return a + b;
}
long long gcd(long long a, long long b) {
  while (a != 0 && b != 0) {
    if (a > b) {
      a %= b;
    } else {
      b %= a;
    }
  }
  return a + b;
}
long long next(long long i) {
  long long a[13], j = 0;
  memset(a, 0, sizeof(a));
  while (i != 0) {
    a[j] = i % 10;
    i /= 10;
    j++;
  }
  bool pp = 0;
  for (int k = j - 1; k >= 0; k--) {
    if (pp) {
      a[k] = 4;
      continue;
    }
    if (a[k] < 4) {
      a[k] = 4;
      pp = 1;
      continue;
    }
    if (4 < a[k] && a[k] < 7) {
      a[k] = 7;
      pp = 1;
      continue;
    }
    if (a[k] > 7) {
      while (a[k + 1] == 7) {
        k++;
      }
      if (a[k + 1] == 4) {
        a[k + 1] = 7;
      } else {
        a[k + 1] = 4;
      }
      a[k] = 4;
      pp = 1;
      continue;
    }
  }
  i = 0;
  for (int j = 12; j >= 0; j--) {
    i = i * 10 + a[j];
  }
  return i;
}
int main() {
  ios::sync_with_stdio(0);
  int n, k;
  string s;
  cin >> n >> k >> s;
  for (int i = 0; i < n - 1; i++) {
    if (i < 0) i = 0;
    if (k <= 0) break;
    if (s[i] == '4' && s[i + 1] == '7') {
      if (i % 2 == 0) {
        if (i + 2 < n && s[i + 2] == '7') {
          if (k % 2 == 1) {
            s[i + 1] = '4';
          }
          k = 0;
          break;
        } else {
          s[i + 1] = '4';
          k--;
        }
      } else {
        s[i] = '7';
        i -= 2;
        k--;
      }
    }
  }
  cout << s;
  return 0;
}
