#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
string s;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
  cin >> s;
  if (s.size() % 2 == 1) {
    int p = s.size() + 1;
    for (int j = 1; j <= p / 2; ++j) cout << 4;
    for (int j = 1; j <= p / 2; ++j) cout << 7;
    return 0;
  }
  int p = s.size();
  int c = 0;
  for (int j = 1; j <= p / 2; ++j)
    if (s[j - 1] - '0' > 7) {
      c = 1;
      break;
    } else if (s[j - 1] - '0' < 7) {
      c = -1;
      break;
    }
  if (c == 0)
    for (int j = p / 2 + 1; j <= p; ++j)
      if (s[j - 1] - '0' > 4) {
        c = 1;
        break;
      } else if (s[j - 1] - '0' < 4) {
        c = -1;
        break;
      }
  if (c == 1) {
    p += 2;
    for (int j = 1; j <= p / 2; ++j) cout << 4;
    for (int j = 1; j <= p / 2; ++j) cout << 7;
    return 0;
  }
  string sol;
  int nr4 = 0, nr7 = 0;
  int n = s.size();
  for (int i = 0; i < s.size(); ++i) {
    if (s[i] < '4') {
      sol += '4', ++nr4;
      break;
    } else if (s[i] == '4') {
      if (nr4 < n / 2)
        sol += '4', ++nr4;
      else {
        sol += '7', ++nr7;
        break;
      }
    } else if (s[i] < '7') {
      sol += '7';
      ++nr7;
      break;
    } else if (s[i] == '7') {
      if (nr7 < n / 2) {
        sol += '7';
        ++nr7;
      } else {
        while (1) {
          if (sol.back() == '4') {
            --nr4, sol.pop_back();
            if (nr7 < n / 2) {
              sol += '7', ++nr7;
              break;
            }
          } else
            --nr7, sol.pop_back();
        }
        break;
      }
    } else {
      while (sol.back() == '7') sol.pop_back(), --nr7;
      if (!sol.empty()) sol.pop_back(), --nr4;
      sol += '7', ++nr7;
      break;
    }
  }
  cout << sol;
  for (int i = nr4 + 1; i <= n / 2; ++i) cout << 4;
  for (int i = nr7 + 1; i <= n / 2; ++i) cout << 7;
  return 0;
}
