#include <iostream>

int main() {
    int n, k, r;
    std::cin >> n >> k;
    if (k == 0 || k > n) {
        r = 0;
    } else if (n + 1 - k == k) {
        r = n - 1;
    } else {
        r = n - 2;
    }
    std::cout << r << std::endl;
}