#include <bits/stdc++.h>
using namespace std;
const int N = int(1e5 + 3);
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string s;
  cin >> s;
  int size = s.length();
  int count4 = 0;
  int count7 = 0;
  for (int i = 0; i < size; i++) {
    if (s[i] == '7') count7++;
    if (s[i] == '4') count4++;
  }
  if (count4 == 0 && count7 == 0)
    cout << -1 << '\n';
  else if (count4 >= count7)
    cout << 4 << '\n';
  else
    cout << 7 << '\n';
}
