#include <iostream>
using namespace std;

int main(){
	int n, k;
	cin >> n >> k;
	if(k < 1 || n < k){
		cout << 0 << endl;
		return 0;
	}
	cout << ((n & 1) && (n + 1) / 2 == k ? n - 1 : n - 2) << endl;
}