#include <bits/stdc++.h>
using namespace std;

int main(){
    unsigned long long l, k, i=0;
    cin >> l >> k;
    if(l%(2*k) == 0){
        i = l/(2*k) -1;
    }else{
        i = l/(2*k);
    }
    cout << i*k << endl ;
    return 0;
}