#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
using namespace std;

int main(){
    int L, K;
    cin >> L >> K;
    int cnt = 0;
    while(true){
        L -= 2*K;
        if(L<=0) break;
        cnt++;
    }
    cout << cnt*K << endl;
}