#include <bits/stdc++.h>
using namespace std;
int main() {
  static string s, t;
  cin >> s;
  int n = s.size();
  if (!(n & 1))
    if (string(n / 2, '7') + string(n / 2, '4') < s)
      cout << string(n / 2 + 1, '4') << string(n / 2 + 1, '7') << endl;
    else {
      int n4 = n / 2, n7 = n / 2;
      bool flag = 0;
      for (int i = 0; i < n; ++i) {
        bool ok = 0;
        if (n4 > 0) {
          if (flag || s[i] < '4')
            ok = 1;
          else if (s[i] == '4')
            for (int j = i + 1; j <= n; ++j) {
              if (j == n) {
                ok = 1;
                break;
              }
              char ch = (j <= i + n7 ? '7' : '4');
              if (ch > s[j]) ok = 1;
              if (ch != s[j]) break;
            }
        }
        if (ok)
          t += "4", --n4;
        else
          t += "7", --n7;
        if (t[i] > s[i]) flag = 1;
      }
      cout << t << endl;
    }
  else
    cout << string(n / 2 + 1, '4') << string(n / 2 + 1, '7') << endl;
  return 0;
}
