public static void main ( String [ ] args ) {
  Scanner scn = new Scanner ( System . in ) ;
  int t = scn . nextInt ( ) ;
  PrintWriter out = new PrintWriter ( System . out ) ;
  A : while ( t -- > 0 ) {
    int n = scn . nextInt ( ) ;
    char a [ ] = scn . next ( ) . toCharArray ( ) ;
    char b [ ] = scn . next ( ) . toCharArray ( ) ;
    int pnt [ ] = new int [ 26 ] ;
    for ( int i = 0 ;
    i < 26 ;
    i ++ ) pnt [ i ] = i ;
    int ans = 0 ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      if ( a [ i ] > b [ i ] ) {
        out . println ( - 1 ) ;
        continue A ;
      }
      int r1 = find ( pnt , a [ i ] - 'a' ) , r2 = find ( pnt , b [ i ] - 'a' ) ;
      if ( r1 != r2 ) {
        ans ++ ;
        pnt [ r1 ] = r2 ;
      }
    }
    out . println ( ans ) ;
  }
  out . close ( ) ;
}

public static void main ( String [ ] args ) { Scanner scn = new Scanner ( System . in ) ; int t = scn . nextInt ( ) ; PrintWriter out = new PrintWriter ( System . out ) ; A : while ( t -- > 0 ) { int n = scn . nextInt ( ) ; char a [ ] = scn . next ( ) . toCharArray ( ) ; char b [ ] = scn . next ( ) . toCharArray ( ) ; int pnt [ ] = new int [ 26 ] ; for ( int i = 0 ; i < 26 ; i ++ ) pnt [ i ] = i ; int ans = 0 ; for ( int i = 0 ; i < n ; i ++ ) { if ( a [ i ] > b [ i ] ) { out . println ( - 1 ) ; continue A ; } int r1 = find ( pnt , a [ i ] - ' a ' ) , r2 = find ( pnt , b [ i ] - ' a ' ) ; if ( r1 != r2 ) { ans ++ ; pnt [ r1 ] = r2 ; } } out . println ( ans ) ; } out . close ( ) ; }
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