#include using namespace std; typedef pair P; typedef pair> PP; typedef long long ll; const double EPS = 1e-8; const int INF = 1e9; const int MOD = 1e9+7; int dy[] = {0,1,0,-1}; int dx[] = {1,0,-1,0}; int main(void) { int L,K; cin >> L >> K; int ans = 0; while(L > 2 * K){ L -= 2 * K; ans += K; } cout << ans << endl; return 0; }