/* -*- coding: utf-8 -*-
 *
 * 2767.cc:  No.2767 Add to Divide - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int INF = 1 << 30;

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  int tn;
  scanf("%d", &tn);

  while (tn--) {
    int a, b;
    scanf("%d%d", &a, &b);

    if (b % a == 0) puts("0");
    else {
      // b+x=k(a+x) [k>=2,x>0] -> b-a=k(a+x)-a-x=(k-1)(a+x)
      int minx = INF, d = b - a;
      for (int p = 1; p * p <= d; p++)
	if (d % p == 0) {
	  int q = d / p;
	  if (p >= a) minx = min(minx, p - a);
	  if (q >= a) minx = min(minx, q - a);
	}
      printf("%d\n", (minx < INF) ? minx : -1);
    }
  }

  return 0;
}