#include <stdio.h>

int main ()
{	
	long int n;
	int a, s=0;
	scanf ( "%ld", &n);
	while ( n > 0 )
	{	
		a = n % 10;
		if ( a > s)
			s = a;
		n = n / 10;
	}
	printf ( "%d\n", s);
	return 0;
}