#include <cstdio>
#include <cmath>
using namespace std;
int main() {
    int n, m;
    double money, hl;
    scanf("%d", &n);
    for (int i = 0; i < n; i++) {
        scanf("%lf%d", &money, &m);
        hl = (m - 2) * ceil(money / (m - 1)) + money / (m - 1) - money;
        if (hl > 0)
            printf("%0.2f\n", hl);
        else
            printf("CSFNB!\n");
    }
    return 0;
}