public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  int T = in . nextInt ( ) ;
  for ( int t = 0 ;
  t < T ;
  t ++ ) {
    String A = in . next ( ) ;
    String B = in . next ( ) ;
    int a = A . length ( ) ;
    int b = B . length ( ) ;
    int gcd = gcd ( a , b ) ;
    int aCopies = b / gcd ;
    int bCopies = a / gcd ;
    StringBuilder aa = new StringBuilder ( ) ;
    StringBuilder bb = new StringBuilder ( ) ;
    for ( int i = 0 ;
    i < aCopies ;
    i ++ ) {
      aa . append ( A ) ;
    }
    for ( int i = 0 ;
    i < bCopies ;
    i ++ ) {
      bb . append ( B ) ;
    }
    boolean ok = aa . toString ( ) . equals ( bb . toString ( ) ) ;
    System . out . println ( ok ? aa : "-1" ) ;
  }
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; int T = in . nextInt ( ) ; for ( int t = 0 ; t < T ; t ++ ) { String A = in . next ( ) ; String B = in . next ( ) ; int a = A . length ( ) ; int b = B . length ( ) ; int gcd = gcd ( a , b ) ; int aCopies = b / gcd ; int bCopies = a / gcd ; StringBuilder aa = new StringBuilder ( ) ; StringBuilder bb = new StringBuilder ( ) ; for ( int i = 0 ; i < aCopies ; i ++ ) { aa . append ( A ) ; } for ( int i = 0 ; i < bCopies ; i ++ ) { bb . append ( B ) ; } boolean ok = aa . toString ( ) . equals ( bb . toString ( ) ) ; System . out . println ( ok ? aa : " - 1" ) ; } }
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