#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
#define double long double
int n, t;
const int N = 1e5 + 5;
double f[N], res;
double get(int a, int b)
{
    return exp((f[b] - f[a] - f[b - a]) - log(2) * (b + 1));
}
int main()
{
    for (int i = 1; i <= 50000; i++)
        f[i] = f[i - 1] + log(i);
    cin >> t;
    while (t--)
    {
        cin >> n, n >>= 1, res = 0.0;
        for (int j = 0; j <= n - 2; j++)
            res += get(j, j + n - 1);
        cout << fixed << setprecision(4) << 2.0 * res << "\n";
    }
}

对着100000测了25组，算编译时间才300ms，这时限3s咋还T了呢