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:
		if A<2:t=1
		D=[1]
		for p in P:
			c=0
			while e%p<1:e//=p;c+=1;
			r=len(D)
			for i in R(c):
				for m in[n*p for n in D[-r:]]:
					D+=[m]
					if A<=m<t:t=m
		r=len(D)
		if e>1:
			for m in[n*e for n in D[-r:]]:
				D+=[m]
				if A<=m<t:t=m
	print([0,max(t-A,-1)][e>0])