#include <bits/stdc++.h>

using namespace std;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)

int main() {

  int L, K; cin >>L>>K;cout<<L/2/K*K<<endl;
  return 0;
}