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