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

int main()
{
	int n;
	cin >> n;
	int a, b, c, d;
	a = n / 1000 % 10;
	b = n / 100 % 10;
	c = n / 10 % 10;
	d = n % 10;
	cout << max(max(a, b), max(c, d));
	return 0;
}