#include<iostream>
#include<stdio.h>

using namespace std;

int main(){

    int l, k;
    cin>> l>> k;
    bool flag = false;

    int cnt = 0;
    while(l>0){
        l -= k*2;
        cnt++;
        flag = true;
    }
    if(true) cnt--;

    cout<< cnt*k<< endl;
    return 0;
}