#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  string s;
  cin >> s;
  int n = s.size();
  if (n % 2 == 1) {
    for (int i = 0; i < n + 1; ++i) {
      cout << (i < (n + 1) / 2 ? 4 : 7);
    }
    cout << endl;
    return 0;
  }
  int seven = 0, four = 0;
  for (int i = 0; i < n; ++i) {
    if (s[i] == '4') {
      four++;
    }
    if (s[i] == '7') {
      seven++;
    }
  }
  if (seven == four && seven + four == n) {
    cout << s << endl;
    return 0;
  }
  for (int i = n - 1; i >= 0; --i) {
    if (s[i] == '4') {
      four--;
    }
    if (s[i] == '7') {
      seven--;
    }
    if (seven + four != i) {
      continue;
    }
    if (seven > n / 2 || four > n / 2) {
      continue;
    }
    if (s[i] < '4' && n / 2 - four > 0) {
      s[i] = '4';
      four++;
      int x = i + 1;
      while (four < n / 2) {
        s[x++] = '4';
        four++;
      }
      while (seven < n / 2) {
        s[x++] = '7';
        seven++;
      }
      cout << s << endl;
      return 0;
    }
    if (s[i] < '7' && n / 2 - seven > 0) {
      s[i] = '7';
      seven++;
      int x = i + 1;
      while (four < n / 2) {
        s[x++] = '4';
        four++;
      }
      while (seven < n / 2) {
        s[x++] = '7';
        seven++;
      }
      cout << s << endl;
      return 0;
    }
  }
  for (int i = 0; i < n + 2; ++i) {
    cout << (i < (n + 2) / 2 ? 4 : 7);
  }
  cout << endl;
  return 0;
}
