#include <bits/stdc++.h>
#define int long long
#define double long double
using namespace std;
const int MOD = 1000000007;
const int INF = 1e14;
using Graph = vector<vector<int>>;

signed main(){
  int L, K;
  cin >> L >> K;
  int ans = L/(2*K)*K;
  if( L%(2*K) == 0 ) ans -= K;
  cout << ans << endl;
}