#include<iostream>
#include<iomanip>
#include<cmath>
#include<string>
#include<algorithm>
#include<vector>
#include<math.h>
#include<stack>
using namespace std;
int main(void){
    long L, K; cin >> L >> K;
    long leng = L /2 + 1;
    int cnt = 0;
    while(true){
        if(leng <= K + 1) break;
        leng -= K;
        cnt++;
    }
    cout << cnt * K << endl;
    return 0;
}