#include <bits/stdc++.h>
using namespace std;
int main() {
  long long b, c, d, e, f, g, h, i, j = 0, k = 2, l, o = 0, m = 0, n = 0, q = 1,
                                    r = 1, s, t, u, v, x = 1, y = 1,
                                    z = 1111111;
  string a;
  cin >> a;
  b = a.size();
  for (i = 0; i < b; i++) {
    if (a[i] == '4') n++;
    if (a[i] == '7') m++;
  }
  if (n == 0 && m == 0) {
    cout << -1;
    return 0;
  }
  if (m > n)
    cout << 7;
  else
    cout << 4;
}
