#include <bits/stdc++.h>
using namespace std;
int main(){
	int a,s,d,f,k;
	cin>>a;
	s=a%10;
	d=a/10%10;
	f=a/100%10;
	k=a/1000;
	if(d<s){
		swap(d,s);
	}
	if(d<f){
		swap(d,f);
	}
	if(d<k){
		swap(d,k);
	}
	cout<<d;
}

/**************************************************************
	Problem: 1718
	User: tonghuawei
	Language: C
	Result: Compile Error
****************************************************************/