#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int l = s.length();
  int fc = 0;
  int sc = 0;
  for (int i = 0; i < l; i++) {
    if (s[i] == '4') fc++;
    if (s[i] == '7') sc++;
  }
  if (!fc && !sc)
    cout << "-1" << endl;
  else if (sc > fc)
    cout << "7" << endl;
  else
    cout << "4" << endl;
  return 0;
}
