#include "bits/stdc++.h"
using namespace std;

int main() {
	int N, K;
	cin >> N >> K;
	if (K == 0 || N < K) cout << 0 << endl;
	else {
		if (K * 2 - 1 == N) cout << N - 1 << endl;
		else cout << N - 2 << endl;
	}
}