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

int main(){
    int t;
    cin>>t;
    while(t--){
        string a,b,c="",d="";
        cin>>a>>b;
        int n=a.size(),m=b.size();
        int x=n*m/__gcd(n,m);
        for(int i=1;i<=x/n;i++){
            c+=a;
        }
        for(int i=1;i<=x/m;i++){
            d+=b;
        }
        if(c==d){
            cout<<c;
        }else{
            cout<<-1;
        }
        cout<<"\n";
    }
}