#include<cstdio>


int main(void)
{
	int n, k, ans;

	while(scanf("%d%d", &n, &k)==2)
	{
		if(k==0 || k>n) ans=0;
		else if(k==n-k+1) ans=n-1;
		else ans=n-2;

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