#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    long long L,K;
    cin >> L >> K;
    
    cout << ( L % (2*K) ? ( L / (2*K) ) * K : (( L / (2*K) )-1 ) * K ) << endl;       
    
    return 0;
}