#include <bits/stdc++.h>
using namespace std;

#define int long long
const int N = 2e5 + 10;

int n, m;

void solve()
{
	cin >> n >> m;
	if (n <= m / 2)	puts("0");
	else if (n == m)	cout << n * 2 << "\n";
	else	cout << (n - 1) * 2 << "\n";
	return ;
}

signed main()
{
	ios::sync_with_stdio(0);
	solve();
	return 0;
}