#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int,int>;
#define rep(i,n) for(int i=0;i<n;i++)

const ll mod = ll(1e9) + 7;
const int INF = 1<<29;

int main(){
    double l,k;
    cin >> l >> k;
    l = l / 2.0;
    double eat = 0;
    while (eat<l-k){
        eat += k;
    }
    printf("%d",int(eat));
    return 0;
}