n=input()
a=-1
f=n.count('4')
s=n.count('7')
if f>0:
a=4
if s>f:
a=7
print(a)
#include<bits/stdc++.h>
using namespace std;
#define ll long long int
int main()
{
string s;
cin>>s;
int totalSeven = 0,totalFour = 0,i;
for(i=0; s[i]; i++)
{
if(s[i]=='4')
totalFour++;
else if(s[i]=='7')
totalSeven++;
}
if(!totalFour && !totalSeven)
cout<<-1<<endl;
else if(totalFour>=totalSeven)
cout<<4<<endl;
else
cout<<7<<endl;
}
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