#include<iostream>

using namespace std;

int main(){
    int L, K, n, d;
    d, n = 0;
    cin >> L >> K;

    while(1){
        if(L - K * 2 > 0){
            L -= K * 2;
            n++;
        }else{
            d = n * K;
            break;
        }
    }

    cout << d << endl;

    return 0;
}