I,R,O=input,range,print
L=9**5
P=[]
C=[0]*L
for i in R(2,L):
	if C[i]<1:
		P+=[i]
		j=i*i
		while j<L:C[j],j=1,j+i
for t in R(int(I())):
	A,B=map(int,I().split())
	t=e=B-A
	if e:
		D=[1]
		for p in P:
			c=0
			while e%p<1:e//=p;c+=1;
			r=len(D)
			for i in R(c):D+=[n*p for n in D[-r:]]
		D.sort()
		if e>1:D+=[e]
		r=len(D)-1
		while r and D[r-1]>=A:r-=1
	print(max(D[r]-A,-1)if e else 0)