#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<math.h>
#include<map>
#include<iomanip>
#include<queue>

const long long INF = 1e6+7;
const long long MOD = 1e9+7;
const double PI=acos(-1);

using namespace std;

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