#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int n;
double a;
cin >> n >> a;
double b = -1;
n -= 2;
for (int i = 1; i <= n; i++)
b = max(b, (-a * (n - i + 1) + i * (n + 1) * (n - i + 1)) / i);
cout << fixed << setprecision(2) << max(b, 0.0) << endl;
return 0;
}
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