#include <iostream>
#include <iomanip>
using namespace std;

int n;
double a, b;

int main() {
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a >> b;
        double temp = b / (a - 1);
        temp = temp - int(temp);
        if (temp < 0.00001)
            cout << "CSFNB!" << endl;
        else
            cout << fixed << setprecision(6) << (a - 2) * temp << endl;
    }
}