#include <stdio.h>
void main()
{
	int n , r , max = 0;
	scanf("%d", &n);
	while(n != 0)
	{
		r = n % 10;
		n = n / 10;
		if(r > max )
		{
			max = r;
		}
	}
	printf("%d\n", max);
}