#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> pl;

int main()
{
	ll N, D,ans;
	cin >> N >> D;

	if (N == D)
		ans = 0;
	else if (N % D == 0)
		ans = N / D - 1;
	else if (N % 2 == 1 || (N % 2 == 0 && D % 2 == 1))
		ans = N - 1;
	else
		ans = N / 2 -1;

	cout << ans << endl;
	return	0;
}