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

int main( int argc, char *argv[] )
{
int N;
int K;
int iAns;

	ios::sync_with_stdio( false );
	cin.tie( 0 );

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

	cout << iAns << endl;

	return 0;
}