#include <bits/stdc++.h>
using namespace std;
string s, ans;
int n;
bool solve(int p, int n4, int n7, bool ok) {
  if (p == n) return 1;
  if (ok) {
    for (int i = 0; i < n7; ++i) ans += '7';
    for (int i = 0; i < n4; ++i) ans += '4';
    return 1;
  }
  if (n4 >= 1 && s[p] <= '4' && solve(p + 1, n4 - 1, n7, s[p] < '4')) {
    ans += '4';
    return 1;
  } else if (n7 >= 1 && s[p] <= '7' && solve(p + 1, n4, n7 - 1, s[p] < '7')) {
    ans += '7';
    return 1;
  }
  return false;
}
int main() {
  while (cin >> s) {
    n = s.size();
    ans.clear();
    if (n % 2 == 0 && solve(0, n / 2, n / 2, false)) {
      reverse(ans.begin(), ans.end());
      cout << ans << endl;
    } else {
      if (n % 2 == 0)
        n += 2;
      else
        ++n;
      for (int i = 0; i < n / 2; ++i) cout << 4;
      for (int i = 0; i < n / 2; ++i) cout << 7;
      cout << endl;
    }
  }
  return 0;
}
