#include <bits/stdc++.h>
using namespace std;
struct bigNum {
  vector<int> val;
  bigNum() { val.push_back(0); }
  void print() {
    printf("%d", val[val.size() - 1]);
    for (int i = val.size() - 2; i >= 0; i--) printf("%05d", val[i]);
  }
  bigNum& operator+=(const bigNum& b) {
    int n = max(val.size(), b.val.size());
    int res = 0;
    for (int i = 0; i < n; i++) {
      if (i < b.val.size()) res += b.val[i];
      if (i < this->val.size())
        res += this->val[i];
      else
        this->val.push_back(0);
      this->val[i] = res % (100000);
      res /= 100000;
    }
    if (res != 0) this->val.push_back(res);
    return *this;
  }
  bigNum& operator=(const string& x) {
    this->val.clear();
    int n = x.length();
    int res = 0;
    for (int i = n - 1; i >= 0; i--) {
      res = res * 10 + (x[i] - '0');
      if (res > 100000) {
        this->val.push_back(res % 100000);
        res /= 100000;
      }
    }
    if (res > 0) this->val.push_back(res % 100000);
    return *this;
  }
  void divide2() {
    bool inc = false;
    int n = this->val.size();
    for (int i = n - 1; i >= 0; i++) {
      int num = this->val[i] + (inc ? 10 : 0);
      this->val[i] = num / 2;
      inc = (num % 2 == 1);
    }
    if (this->val[n - 1] == 0) this->val.erase(this->val.end() - 1);
  }
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t;
  cin >> t;
  while (t--) {
    long long x, y, p, q;
    cin >> x >> y >> p >> q;
    if (p == 0) {
      cout << (x == 0 ? "0\n" : "-1\n");
      continue;
    }
    if (p == q) {
      cout << (x == y ? "0\n" : "-1\n");
      continue;
    }
    long long low = 1, high = 9999999999;
    bool poss = false;
    while (low < high) {
      long long mid = (low + high) >> 1;
      if (q * mid >= y && p * mid >= x && q * mid - y >= p * mid - x) {
        high = mid;
        poss = true;
      } else
        low = mid + 1;
    }
    if (poss)
      cout << q * low - y << endl;
    else
      cout << "-1\n";
  }
  return 0;
}
