#include <bits/stdc++.h>
int take() {
  int n;
  scanf("%d", &n);
  return n;
}
double ttake() {
  double n;
  scanf("%lf", &n);
  return n;
}
long long takes() {
  long long n;
  scanf("%lld", &n);
  return n;
}
int cas;
using namespace std;
bool approximatelyEqual(float a, float b, float epsilon) {
  return fabs(a - b) <= ((fabs(a) < fabs(b) ? fabs(b) : fabs(a)) * epsilon);
}
bool essentiallyEqual(float a, float b, float epsilon) {
  return fabs(a - b) <= ((fabs(a) > fabs(b) ? fabs(b) : fabs(a)) * epsilon);
}
bool definitelyGreaterThan(float a, float b, float epsilon) {
  return (a - b) > ((fabs(a) < fabs(b) ? fabs(b) : fabs(a)) * epsilon);
}
bool definitelyLessThan(float a, float b, float epsilon) {
  return (b - a) > ((fabs(a) < fabs(b) ? fabs(b) : fabs(a)) * epsilon);
}
int main() {
  int tc = 1;
  while (tc--) {
    string s, ans = "";
    cin >> s;
    int f = 0, se = 0;
    for (int i = 0; i <= s.size() - 1; i += 1) {
      s[i] == '4' ? f++ : s[i] == '7' ? se++ : f, se;
    }
    if (!f && !se)
      printf("-1\n");
    else {
      if (f >= se)
        printf("4\n");
      else
        printf("7\n");
    }
  }
  { return 0; };
}
