#include <iostream>
using namespace std;
int main(){
	int N, K;
	cin >> N >> K;
	if (K == 0){
		cout << 0 << endl;
	} else if (K > N){
		cout << 0 << endl;
	} else if (N == K * 2 - 1){
		cout << N - 1 << endl;
	} else {
		cout << N - 2 << endl;
	}
}