#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <stack>
#include <queue>
#include <set>
#include <map>
using namespace std;

int main(){
	int n, k;
	cin >> n >> k;

	if (k > n || k <= 0) {
		cout << 0 << endl;
	}
	else if((n  + 1)/ 2 == k && n % 2 == 1){
		cout << n - 1 << endl;
	}
	else {
		cout << n - 2 << endl;
	}
}