#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007

using namespace std;


int main(void) {
	long long int l, k, total = 0;
	cin >> l >> k;
	while (l > 2 * k) {
		total += k;
		l -= 2 * k;
	}
	cout << total<< endl;
	return 0;
}