// BY : VIDIT BHARDWAJ   #include<bits/stdc++.h>   using namespace std;      #define int                 long long int   #define ff                  first   #define ss                  second   #define pb                  push_back   #define popb                pop_back   #define lb                  lower_bound   #define ub                  upper_bound   #define all(v)              (v).begin(),(v).end()   #define tc(t)               int t; cin>>t; while(t--)   #define pii                 pair<int,int>   #define max_pq              priority_queue<int>   #define min_pq              priority_queue<int,vector<int>, greater<int> >   #define fast_ios            ios_base::sync_with_stdio(false);cin.tie(NULL);   #define ps(x,y)             fixed<<setprecision(y)<<x    #define setbits(x)          __builtin_popcountll(x)   #define zerobits(x)         __builtin_ctzll(x)   #define YES                 cout<< YES <<  n    #define NO                  cout<< NO <<  n    #define Yes                 cout<< Yes <<  n    #define No                  cout<< No <<  n    #define endl                  n               const int N=200010;   const long long mod=1000000007;   // const long long mod=998244353;   const long long inf=1000000000000000000;   const long double pi=3.14159265358979323846264338;      /*_______________________________________________________________________*/      signed main(){   #ifndef ONLINE_JUDGE       freopen( input.txt ,  r , stdin);       freopen( output.txt ,  w , stdout);   #endif       fast_ios       tc(t){           string str,s; cin>>str>>s;           int n=str.size(),m=s.size();              if(n<m){            string temp=str;            str=s,s=temp;            n=str.size(),m=s.size();           }              if(s!=str.substr(0,s.size())){            cout<< -1 <<endl;            continue;           }           int it=500,yes=0;        string temp=s;        while(it>0){         if(s.size()%str.size()==0){          yes=1;          break;         }         s+=temp,it--;        }        if(yes){         temp=str;         int x=s.size()/str.size();         for(int i=1;i<x;i++){          str+=temp;         }         if(str==s) cout<<s<<endl;         else cout<< -1 <<endl;        }        else cout<< -1 <<endl;       }          return 0;   }   /*      */