#include <bits/stdc++.h>
using namespace std;
long long t, x, y, p, q;
bool check(long long mid) {
  long long pp = p * mid;
  long long qq = q * mid;
  if (pp - x < 0 || qq - y < 0) return false;
  return (pp - x <= qq - y);
}
int main() {
  scanf("%lld", &t);
  while (t--) {
    scanf("%lld %lld %lld %lld", &x, &y, &p, &q);
    if (x * q == p * y) {
      printf("0\n");
      continue;
    }
    long long lo = 1, hi = 2000000000;
    while (lo + 1 < hi) {
      long long mid = (lo + hi) / 2LL;
      if (check(mid))
        hi = mid;
      else
        lo = mid;
    }
    if (check(lo)) hi = lo;
    if (check(hi))
      printf("%lld\n", hi * q - y);
    else
      printf("-1\n");
  }
  return 0;
}
