public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  PrintWriter out = new PrintWriter ( System . out ) ;
  int N = Integer . parseInt ( sc . next ( ) ) ;
  String S = sc . next ( ) ;
  int [ ] cntnum = new int [ N ] ;
  int cnt = 0 ;
  for ( int i = 1 ;
  i < S . length ( ) ;
  i ++ ) {
    int ans = 0 ;
    String s1 = S . substring ( 0 , i ) ;
    String s2 = S . substring ( i , N ) ;
    for ( char c = 'a' ;
    c <= 'z' ;
    c ++ ) {
      String d = String . valueOf ( c ) ;
      if ( s1 . indexOf ( d ) != - 1 && s2 . indexOf ( d ) != - 1 ) {
        ans ++ ;
      }
    }
    if ( cnt < ans ) {
      cnt = ans ;
    }
  }
  out . print ( cnt ) ;
  out . flush ( ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; PrintWriter out = new PrintWriter ( System . out ) ; int N = Integer . parseInt ( sc . next ( ) ) ; String S = sc . next ( ) ; int [ ] cntnum = new int [ N ] ; int cnt = 0 ; for ( int i = 1 ; i < S . length ( ) ; i ++ ) { int ans = 0 ; String s1 = S . substring ( 0 , i ) ; String s2 = S . substring ( i , N ) ; for ( char c = ' a ' ; c <= ' z ' ; c ++ ) { String d = String . valueOf ( c ) ; if ( s1 . indexOf ( d ) != - 1 && s2 . indexOf ( d ) != - 1 ) { ans ++ ; } } if ( cnt < ans ) { cnt = ans ; } } out . print ( cnt ) ; out . flush ( ) ; }
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