#include <bits/stdc++.h>
using namespace std;
string get(int n, int m) {
  string str;
  for (int i = 1; i <= n; i++) str.push_back('4');
  for (int i = 1; i <= m; i++) str.push_back('7');
  return str;
}
int32_t main() {
  ios_base ::sync_with_stdio(0);
  cin.tie(0);
  string str;
  cin >> str;
  int n = str.size();
  if (n & 1) {
    cout << get((n + 1) / 2, (n + 1) / 2) << endl;
    return 0;
  }
  int cnta = 0, cntb = 0, ans = -1, aa = 0, bb = 0;
  for (int i = 0; i < n; i++) {
    if (cnta + 1 <= n / 2 && str[i] < '7') {
      ans = i;
      aa = cnta;
      bb = cntb;
    }
    if (cntb + 1 <= n / 2 && str[i] < '4') {
      ans = i;
      aa = cnta;
      bb = cntb;
    }
    cnta += str[i] == '7';
    cntb += str[i] == '4';
    if ((str[i] != '4' && str[i] != '7') || cnta > n / 2 || cntb > n / 2) break;
  }
  if (cnta + cntb == n && cnta == cntb) {
    cout << str << endl;
    return 0;
  }
  if (ans == -1) {
    cout << get((n + 2) / 2, (n + 2) / 2) << endl;
    return 0;
  }
  string x;
  for (int i = 0; i < ans; i++) {
    x.push_back(str[i]);
  }
  if (bb + 1 <= n / 2 && str[ans] < '4') {
    ++bb;
    x.push_back('4');
  } else {
    ++aa;
    x.push_back('7');
  }
  x += get(n / 2 - bb, n / 2 - aa);
  cout << x << endl;
}
