#include <stdio.h>
int main(void){
	int n, k;
	scanf("%d%d", &n, &k);
	if(k == 0 || k > n){
		printf("0\n");
	}else{
		if(k * 2 - 1 == n){
			printf("%d\n", n-1);
		}else{
			printf("%d\n", n-2);
		}
	}
	return 0;
}