#include <bits/stdc++.h>
using namespace std;
int main() {
  long long k4, k7, i, l;
  char s[100];
  long long n;
  gets(s);
  l = strlen(s);
  k4 = 0;
  k7 = 0;
  for (i = 0; i < l; i++) {
    if (s[i] == '4') k4++;
    if (s[i] == '7') k7++;
  }
  if (k4 == 0 && k7 == 0)
    cout << -1;
  else if (k7 > k4)
    cout << 7;
  else
    cout << 4;
  return 0;
}
