#include<iostream>
using namespace std;

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