public static void main ( String args [ ] ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  int k = sc . nextInt ( ) , st = 0 , sum = 0 ;
  int arr [ ] = new int [ 26 ] ;
  String str = sc . next ( ) ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    arr [ str . charAt ( i ) - 97 ] += 1 ;
  }
  for ( int i = 0 ;
  i < 26 ;
  i ++ ) {
    if ( arr [ i ] > 0 ) {
      st += 1 ;
      sum += ( i + 1 ) ;
      if ( st == k ) break ;
      i += 1 ;
    }
  }
  if ( st == k ) System . out . println ( sum ) ;
  else System . out . println ( - 1 ) ;
  sc . close ( ) ;
}

public static void main ( String args [ ] ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; int k = sc . nextInt ( ) , st = 0 , sum = 0 ; int arr [ ] = new int [ 26 ] ; String str = sc . next ( ) ; for ( int i = 0 ; i < n ; i ++ ) { arr [ str . charAt ( i ) - 97 ] += 1 ; } for ( int i = 0 ; i < 26 ; i ++ ) { if ( arr [ i ] > 0 ) { st += 1 ; sum += ( i + 1 ) ; if ( st == k ) break ; i += 1 ; } } if ( st == k ) System . out . println ( sum ) ; else System . out . println ( - 1 ) ; sc . close ( ) ; }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (type_identifier) name: (identifier) dimensions: (dimensions))) 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: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal)) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (decimal_integer_literal))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (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 (expression_statement (assignment_expression left: (array_access array: (identifier) index: (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) right: (decimal_integer_literal))) right: (decimal_integer_literal))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (decimal_integer_literal)) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (decimal_integer_literal))) consequence: (block (expression_statement (assignment_expression left: (identifier) right: (decimal_integer_literal))) (expression_statement (assignment_expression left: (identifier) right: (parenthesized_expression (binary_expression left: (identifier) right: (decimal_integer_literal))))) (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (identifier))) consequence: (break_statement)) (expression_statement (assignment_expression left: (identifier) right: (decimal_integer_literal))))))) (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (identifier))) consequence: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier)))) alternative: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (unary_expression operand: (decimal_integer_literal)))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))