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

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