#include <bits/stdc++.h>

using ll = long long;
using ull = unsigned long long;
using ld = long double;
#define rep(i,a,b) for (int i = a; i < b; i++)
#define irep(i,a,b) for (int i = a; i > b; i--)
#define print(n) cout << n << endl
#define rup(a,b) (a+b-1)/b

using namespace std;

int main(){
    cout << fixed << setprecision(15);
    
    int L,K;
    cin>>L>>K;
    int k  = 2 * K;
    if (L%k == 0) L --;
    print(K*(L/k));
    
    //system("pause");
    return 0;
}