//#include <atcoder/all>
#include <bits/stdc++.h>
#include <cmath>
using namespace std;
//using namespace atcoder;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<ll>>;
Graph G;
vll E;
vector<bool> seen;

int main() {
	ll N,K;
	cin>>N>>K;
	cout<<(N-1)/(2*K)*K<<endl;
	
}