#include <iostream>
#include <math.h>
using namespace std;

int main() {
    int i;
    cin >> i;
    int j[i], k[i];
    double w[i];
    for (int n = 0; n < i; n++) {
        cin >> j[n] >> k[n];
        w[n] = (floor((float)k[n] / j[n] + 0.5f) * j[n] - k[n]) / (j[n] - 1);
    }
    for (int n = 0; n < i; n++) {
        if (w[n] <= 0) {
            cout << "CSFNB!" << endl;
        } else {
            printf("%.6f\n", w[n]);
        }
    }

    return 0;
}