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

int main()
{
	long long N, K;
	cin >> N >> K;

	long long ans = 0;
	if( 1 <= K && K <= N ) {
		ans = N - 2;
		K--;
		if( 1 + K == N - K ) ans = N - 1;
	}

	cout << ans << endl;
}