#include <bits/stdc++.h>
using namespace std;
int n;
string s, t, a;
void dfs(int i, int c4, int c7, bool d) {
  if (c4 + n - i < n / 2 || c7 + n - i < n / 2) return;
  if (i == n) {
    a = t;
    return;
  }
  if (d) {
    t[i] = '4';
    dfs(i + 1, c4 + 1, c7, 1);
    if (a != "") return;
    t[i] = '7';
    dfs(i + 1, c4, c7 + 1, 1);
  } else {
    if (s[i] <= '4') {
      t[i] = '4';
      dfs(i + 1, c4 + 1, c7, s[i] < '4');
      if (a != "") return;
    }
    if (s[i] <= '7') {
      t[i] = '7';
      dfs(i + 1, c4, c7 + 1, s[i] < '7');
    }
  }
}
int main() {
  cin >> s;
restart:
  if (s.size() % 2) s = "0" + s;
  n = s.size();
  t = string(n, '0');
  dfs(0, 0, 0, 0);
  if (a == "") {
    s = "00" + s;
    goto restart;
  } else {
    cout << a << endl;
  }
}
