import math
def lcm(s, k):
res = ""
while k > 0:
res += s
k -= 1
return res
for _ in range(int(input())):
s = input()
t = input()
n, m = len(s), len(t)
g = math.gcd(n, m)
x = lcm(s, m // g)
y = lcm(t, n // g)
if x == y:
print(x)
else:
print(-1)#include<bits/stdc++.h>
using namespace std;
int main()
{
long long tt;
cin>>tt;
while(tt--)
{
long long m,n,i,j,k,l,p,q,r,lcm;
string s1,s2,s3="",s4="";
cin>>s1>>s2;
p=s1.size();
q=s2.size();
r=__gcd(p,q);
lcm=(p*q)/r;
for(i=0;i<lcm/p;i++)
{
s3=s3+s1;
}
for(i=0;i<lcm/q;i++)
{
s4=s4+s2;
}
if(s3==s4)
{
cout<<s3<<endl;
}
else
cout<<-1<<endl;
}
}
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