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

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

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

    return 0;
}