#include <stdio.h>

int main(){
	int x, max;

	scanf("%d", &x);
	if(x<0)
		x*=-1;

	if(x<1000 || x>9999){
		printf("-1\n");
		return 1;
	}

	max = x%10;
	if((x/10)%10>max)
		max = (x/10)%10;
	if((x/100)%10>max)
		max = (x/100)%10;
	if((x/1000)%10>max)
		max = (x/1000)%10;

	printf("%d\n",max);

	return 0;
}