public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int size = sc . nextInt ( ) ;
  char [ ] arr = sc . next ( ) . toCharArray ( ) ;
  int out = 0 ;
  boolean [ ] letters = new boolean [ 26 ] ;
  if ( size > 26 ) {
    System . out . println ( - 1 ) ;
    return ;
  }
  for ( int i = 0 ;
  i < size ;
  i ++ ) {
    if ( letters [ arr [ i ] - 'a' ] == true ) out ++ ;
    else {
      letters [ arr [ i ] - 'a' ] = true ;
    }
  }
  System . out . println ( out ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int size = sc . nextInt ( ) ; char [ ] arr = sc . next ( ) . toCharArray ( ) ; int out = 0 ; boolean [ ] letters = new boolean [ 26 ] ; if ( size > 26 ) { System . out . println ( - 1 ) ; return ; } for ( int i = 0 ; i < size ; i ++ ) { if ( letters [ arr [ i ] - ' a ' ] == true ) out ++ ; else { letters [ arr [ i ] - ' a ' ] = true ; } } System . out . println ( out ) ; }
(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 (field_access object: (identifier) field: (identifier)))))) (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: (array_type element: (integral_type) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) (local_variable_declaration type: (array_type element: (boolean_type) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (boolean_type) dimensions: (dimensions_expr (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))))) (return_statement))) (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 (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal))) right: (true))) consequence: (expression_statement (update_expression (identifier))) alternative: (block (expression_statement (assignment_expression left: (array_access array: (identifier) index: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal))) right: (true))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier))))))