#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define itn int
#define rep2(i,a,n) for(int i = (a);i < (n);i++)
#define rep(i,n) rep2(i,0,n)
using P = pair<int,int>;

int main(void){
    int l,k;
    cin >> l >> k;
    int ans = k * ((l / 2) / k);
    if(((l / 2) % k) == 0)ans -= k;
    cout << ans << endl;
}