#include <bits/stdc++.h>

using namespace std;

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

    int ans;
    if (k < 1 or k > n) {
        ans = 0;
    } else if (n == 2 * k - 1) {
        ans = n - 1;
    } else {
        ans = n - 2;
    }

    cout << ans << endl;
    return 0;
}