#include<bits/stdc++.h> using namespace std; using ll = long long; void solve() { int c4=0,c7=0; string s; cin>>s; for(int i=0; i<s.size(); ++i) { if(s[i]=='4') c4++; else if(s[i]=='7') c7++; } if(c4==0 && c7==0) cout<<"-1"<<endl; else if(c4>c7) cout<<"4"<<endl; else if(c7>c4) cout<<"7"<<endl; else if(c7==c4) cout<<"4"<<endl; } int main() { ios_base::sync_with_stdio(false); cin.tie(nullptr); int TC = 1; //cin >> TC; //cin.ignore(); while (TC--) solve(); }