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

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int l,k;cin >> l >> k;
	int u=l/2;
	u/=k;
	if(u*k*2!=l) cout << u*k << endl;
	else cout << (u-1)*k << endl;
}