#include<bits/stdc++.h>
using namespace std;
int main(){
    int n,g,s,b,q;
    cin>>n;
    g=n%10;
    s=n/10%10;
    b=n/100%10;
    q=n/1000%10;
    if(g>s) swap(g,s);
    if(g>b) swap(g,b);
    if(g>q) swap(g,q);
    if(s>b) swap(s,b);
    if(s>q) swap(s,q);
    if(b>q) swap(b,q);
    cout<<q;
}
/**************************************************************
	Problem: 1718
	User: linmiaoling
	Language: C++
	Result: Accepted
	Time:26 ms
	Memory:2072 kb
****************************************************************/