public static void main ( String [ ] args ) {
  FastScanner fs = new FastScanner ( ) ;
  int n = fs . nextInt ( ) ;
  String s = fs . next ( ) ;
  Map < Character , Boolean > mp = new HashMap < > ( ) ;
  int change = 0 ;
  if ( n > 26 ) {
    System . out . println ( - 1 ) ;
  }
  else {
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      if ( mp . containsKey ( s . charAt ( i ) ) ) {
        change ++ ;
      }
      else {
        mp . put ( s . charAt ( i ) , true ) ;
      }
    }
    System . out . println ( change ) ;
  }
}

public static void main ( String [ ] args ) { FastScanner fs = new FastScanner ( ) ; int n = fs . nextInt ( ) ; String s = fs . next ( ) ; Map < Character , Boolean > mp = new HashMap < > ( ) ; int change = 0 ; if ( n > 26 ) { System . out . println ( - 1 ) ; } else { for ( int i = 0 ; i < n ; i ++ ) { if ( mp . containsKey ( s . charAt ( i ) ) ) { change ++ ; } else { mp . put ( s . charAt ( i ) , true ) ; } } System . out . println ( change ) ; } }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (generic_type (type_identifier) (type_arguments (type_identifier) (type_identifier))) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (generic_type (type_identifier) (type_arguments)) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (decimal_integer_literal))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (unary_expression operand: (decimal_integer_literal)))))) alternative: (block (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))))) consequence: (block (expression_statement (update_expression (identifier)))) alternative: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) (true)))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier))))))))