#include <stdio.h>

int main(void)
{
	int num;
	int dig;
	scanf("%d", &num);
	dig = 0;
	if (num < 0)
	{
		num = -num;
	} 
	while (num != 0)
	{
		int rem = num % 10;
		if (rem > dig)
		{
			dig=rem;
		}
		num = num / 10;
	}
	printf("%d\n", dig);
	return 0;
}