#include <stdio.h>
int main() {
    int x;
    scanf("%d", &x);
    double a[30], b[30], c[30];
    int i;
    for (i = 0; i < x; i++) scanf("%lf %lf", &a[i], &b[i]);
    for (i = 0; i < x; i++) {
        double d, e, f;
        d = b[i] / (a[i] - 1);
        e = (int)(d + 0.5) - d;
        f = e * (a[i] - 2);
        if (f > 0)
            printf("%lf\n", f);
        else
            printf("CSFNB!");
    }
    return 0;
}