#include <bits/stdc++.h>
using namespace std;

int main() {
	int a[4],n;
	cin>>n;
	a[0] = n / 1000;
	a[1] = n / 100 % 10;
	a[2] = n / 10 % 10;
	a[3] = n % 10;
	
	sort(a,a+4);
	
	cout<<a[3];
}

/**************************************************************
	Problem: 1718
	User: admin
	Language: C++
	Result: Accepted
	Time:30 ms
	Memory:2076 kb
****************************************************************/