#include <bits/stdc++.h>
using namespace std;
long long pwr(long long base, long long p, long long mod = (1000000007LL)) {
  long long ans = 1;
  while (p) {
    if (p & 1) ans = (ans * base) % mod;
    base = (base * base) % mod;
    p /= 2;
  }
  return ans;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long nCr(long long n, long long r) {
  long long C[r + 1];
  memset(C, 0, sizeof(C));
  C[0] = 1;
  for (long long i = 1; i <= n; i++)
    for (long long j = min(i, r); j > 0; j--) C[j] = C[j] + C[j - 1];
  return C[r];
}
bool isPrime(long long n) {
  if (n <= 1) return false;
  if (n == 2) return true;
  if (n % 2 == 0) return false;
  long long m = sqrt(n);
  for (long long i = 3; i <= m; i += 2)
    if (n % i == 0) return false;
  return true;
}
long long M(long long a) { return (a % (1000000007LL)); }
bool isPowerOfTwo(long long x) { return x && (!(x & (x - 1))); }
int x, y, p, q;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(0);
  int t;
  cin >> t;
  while (t--) {
    cin >> x >> y >> p >> q;
    if (x == 0 && p == 0) {
      cout << 0 << "\n";
      continue;
    } else if (x == y && p == q) {
      cout << 0 << "\n";
      continue;
    } else if (p == q && x != y) {
      cout << -1 << "\n";
      continue;
    } else if (p == 0 && x != 0) {
      cout << -1 << "\n";
      continue;
    }
    int l = 1, r = 1e9 + 1;
    int mid;
    int f = (x + p - 1) / p;
    int s = (y - x + q - p - 1) / (q - p);
    while (r > l) {
      mid = (l + r) >> 1;
      if (mid >= f && mid >= s) {
        r = mid;
      } else {
        l = mid + 1;
      }
      if (((l + r) >> 1) == mid) {
        break;
      }
    }
    if (mid >= f && mid >= s) {
      cout << (1LL * q * 1LL * mid - 1LL * y * 1LL) << "\n";
    } else if (l >= f && l >= s) {
      cout << (1LL * q * 1LL * l - 1LL * y * 1LL) << "\n";
    } else if (r >= f && r >= s) {
      cout << (1LL * q * 1LL * r - 1LL * y * 1LL) << "\n";
    } else
      cout << -1 << "\n";
  }
  return 0;
}
