#include <bits/stdc++.h>

using ll = long long;

using namespace std;

int main()
{
    ll l,k;
    scanf("%lld", &l);
    scanf("%lld", &k);
    ll k2=k<<1;
    l = ((l+k2-1)/k2)*k2;
    printf("%lld\n",l/2-k);
    return 0;
}