#include <stdio.h>

int main(){
	int x,j,d,s,h,max;
	
	scanf("%d", &x);

	if(x<0)
		x*=-1;
		
	if(x<1000 || x>9999){
		printf("-1\n");
		return 1;
	}

	j = x%10;
	d = (x/10)%10;
	s = (x/100)%10;
	h = x/1000;

	max = j;
	if(d>max)
		max=d;
	if(s>max)
		max=s;
	if(h>max)
		max=h;

	printf("%d\n",max);
	
	return 0;
}