#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using P = pair<int, int>;
using namespace std;

int main() {
    int L, K;
    cin >> L >> K;

    if (K*2 >= L or (L/(K*2)==1 and L%(K*2)==0)) {
        cout << 0 << endl;
    }
    else {
        if (L%(K*2) == 0) {
            cout << K*(L/(K*2)-1) << endl;
        }
        else {
            cout << K*(L/(K*2)) << endl;
        }
    }
}