#include <iostream>

int main () {
    int l, k;
    int yu = 0;

    std::cin >> l >> k;

    if ( k * 2 >= l) {
        std::cout << 0 << std::endl;
    }
    else if (k == 1) {
        yu = (l / (k * 2)) * k - 1;
        std::cout << yu << std::endl;
    }
    else {
        yu = (l / (k * 2)) * k;
        std::cout << yu << std::endl;
    }

    return 0;
}