public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  sc . nextLine ( ) ;
  String input = sc . nextLine ( ) ;
  int count = 0 ;
  if ( input . length ( ) > 26 ) {
    System . out . println ( - 1 ) ;
    return ;
  }
  else {
    int [ ] arr = new int [ 27 ] ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      char temp = input . charAt ( i ) ;
      arr [ temp - 97 ] ++ ;
    }
    for ( int i = 0 ;
    i < arr . length ;
    i ++ ) {
      if ( arr [ i ] != 0 ) {
        count += arr [ i ] - 1 ;
      }
    }
  }
  System . out . println ( count ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; sc . nextLine ( ) ; String input = sc . nextLine ( ) ; int count = 0 ; if ( input . length ( ) > 26 ) { System . out . println ( - 1 ) ; return ; } else { int [ ] arr = new int [ 27 ] ; for ( int i = 0 ; i < n ; i ++ ) { char temp = input . charAt ( i ) ; arr [ temp - 97 ] ++ ; } for ( int i = 0 ; i < arr . length ; i ++ ) { if ( arr [ i ] != 0 ) { count += arr [ i ] - 1 ; } } } System . out . println ( count ) ; }
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