#include<bits/stdc++.h>
using namespace std;
#define NUM 101
int p[NUM];
int main(){
    long long L, K; cin >> L >> K;
    long cnt = 0;
    for(;;){
        if(L <= K* 2) break;
        L -= K * 2;
        cnt++;
        
    }
    cout << K * cnt << endl;
    return 0;
}