#include<iostream>
#include<iomanip>
#include<cmath>
#include<string>
#include<algorithm>
#include<vector>
#include<math.h>
#include<stack>
#include <typeinfo>  
using namespace std;
int main(void){
    long  L, K; cin >> L >> K;
    long len = L / 2;
    long cnt = 0;
    for(;;){
        len -= K;
        if(len <= 0) break;
        cnt++;
        
    }

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