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

int main()
{
int N;
int K;

	cin >> N;
	cin >> K;
	if( K <= 0 || K > N )
		cout << 0 << endl;
	else if( 2 * K - 1 == N )
		cout << N - 1 << endl;
	else
		cout << N - 2 << endl;

	return 0;
}