public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int N = sc . nextInt ( ) ;
  String s = sc . next ( ) ;
  int max = 0 ;
  int a , j ;
  ArrayList < Integer > c = new ArrayList < > ( ) ;
  ArrayList < Integer > cc = new ArrayList < > ( ) ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    j = 0 ;
    a = 0 ;
    while ( j <= i ) {
      int b = s . charAt ( j ) ;
      c . add ( b ) ;
      j ++ ;
    }
    while ( j < N ) {
      int d = s . charAt ( j ) ;
      if ( c . contains ( d ) && ! cc . contains ( d ) ) {
        a ++ ;
        cc . add ( d ) ;
      }
      j ++ ;
    }
    if ( a > max ) max = a ;
    c . clear ( ) ;
    cc . clear ( ) ;
  }
  System . out . println ( max ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int N = sc . nextInt ( ) ; String s = sc . next ( ) ; int max = 0 ; int a , j ; ArrayList < Integer > c = new ArrayList < > ( ) ; ArrayList < Integer > cc = new ArrayList < > ( ) ; for ( int i = 0 ; i < N ; i ++ ) { j = 0 ; a = 0 ; while ( j <= i ) { int b = s . charAt ( j ) ; c . add ( b ) ; j ++ ; } while ( j < N ) { int d = s . charAt ( j ) ; if ( c . contains ( d ) && ! cc . contains ( d ) ) { a ++ ; cc . add ( d ) ; } j ++ ; } if ( a > max ) max = a ; c . clear ( ) ; cc . clear ( ) ; } System . out . println ( max ) ; }
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