#include <bits/stdc++.h>
using namespace std;
unsigned long long min(unsigned long long x, unsigned long long y) {
  if (x < y) return x;
  return y;
}
unsigned long long max(unsigned long long x, unsigned long long y) {
  if (x < y) return y;
  return x;
}
long long min(long long x, long long y) {
  if (x < y) return x;
  return y;
}
long long max(long long x, long long y) {
  if (x < y) return y;
  return x;
}
double min(double x, double y) {
  if (x < y) return x;
  return y;
}
double max(double x, double y) {
  if (x < y) return y;
  return x;
}
unsigned long long gcd(unsigned long long x, unsigned long long y) {
  if (!x) return y;
  if (!y) return x;
  if (x > y) swap(x, y);
  return gcd(x, y % x);
}
unsigned long long inv(unsigned long long a, unsigned long long c) {
  if (a == 1) {
    return 1;
  }
  return ((c - (c / a)) * inv(c % a, c)) % c;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t;
  cin >> t;
  while (t--) {
    long long x, y, p, q;
    cin >> x >> y >> p >> q;
    if (p == q) {
      if (x == y) {
        cout << "0\n";
      } else {
        cout << "-1\n";
      }
      continue;
    }
    if (p == 0) {
      if (x == 0) {
        cout << "0\n";
      } else {
        cout << "-1\n";
      }
      continue;
    }
    long long g = max((y - x + q - p - 1) / (q - p),
                      max((y + q - 1) / q, (x + p - 1) / p));
    cout << (q * g - y) << "\n";
  }
}
