java版

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int T = sc.nextInt();
        while(T-- > 0) {
            long n = sc.nextLong();
            long d = sc.nextLong();
            long x = sc.nextLong();
            long y = sc.nextLong();
            Long a = new Long();
            Long b = new Long();
            long gcd = exgcd(n, d, a, b);
            if((y - x) % gcd != 0) {
                System.out.println("Impossible");
            }else {
                b.v *= (y - x) / gcd;
                n /= gcd;
                System.out.println((b.v % n + n) % n);
            }
        }
        sc.close();
    }

    private static long exgcd(long a, long b, Long x, Long y) {
        if(b == 0) {
            x.v = 1;
            y.v = 0;
            return a;
        }
        long d = exgcd(b, a % b, y, x);
        y.v -= a / b * x.v;
        return d;
    }
}
class Long{
    long v;

    public Long() {};

    public Long(long v) {
        this.v=v;
    }
}