#include<bits/stdc++.h>
using namespace std;
int n,a,b,c,d,maxx;
int main(){
cin>>n;
a=n/1000;
b=n/100%10;
c=n/10%10;
d=n%10;
maxx=a;
if(b>maxx)maxx=b;
if(c>maxx)maxx=c;
if(d>maxx)maxx=d;
cout<<maxx;
return 0;
}