public static void main ( String [ ] args ) throws IOException {
  bufferedReader = new BufferedReader ( new InputStreamReader ( System . in ) ) ;
  int a1 = Integer . parseInt ( next ( ) ) ;
  int a2 = Integer . parseInt ( next ( ) ) ;
  int a3 = Integer . parseInt ( next ( ) ) ;
  int a4 = Integer . parseInt ( next ( ) ) ;
  int d = a2 - a1 ;
  if ( d == a3 - a2 && d == a4 - a3 ) {
    System . out . println ( a4 + d ) ;
  }
  else if ( a1 * a3 == a2 * a2 && a2 * a4 == a3 * a3 && a4 * a4 % a3 == 0 ) {
    System . out . println ( ( a4 * a4 ) / a3 ) ;
  }
  else {
    System . out . println ( 42 ) ;
  }
}

public static void main ( String [ ] args ) throws IOException { bufferedReader = new BufferedReader ( new InputStreamReader ( System . in ) ) ; int a1 = Integer . parseInt ( next ( ) ) ; int a2 = Integer . parseInt ( next ( ) ) ; int a3 = Integer . parseInt ( next ( ) ) ; int a4 = Integer . parseInt ( next ( ) ) ; int d = a2 - a1 ; if ( d == a3 - a2 && d == a4 - a3 ) { System . out . println ( a4 + d ) ; } else if ( a1 * a3 == a2 * a2 && a2 * a4 == a3 * a3 && a4 * a4 % a3 == 0 ) { System . out . println ( ( a4 * a4 ) / a3 ) ; } else { System . out . println ( 42 ) ; } }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) (throws (type_identifier)) body: (block (expression_statement (assignment_expression left: (identifier) right: (object_creation_expression type: (type_identifier) arguments: (argument_list (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 (method_invocation 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 (method_invocation 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 (method_invocation 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 (method_invocation name: (identifier) arguments: (argument_list)))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (identifier) right: (identifier)))) (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (binary_expression left: (identifier) right: (identifier))) right: (binary_expression left: (identifier) right: (binary_expression left: (identifier) right: (identifier))))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (identifier) right: (identifier)))))) alternative: (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (binary_expression left: (identifier) right: (identifier))) right: (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (binary_expression left: (identifier) right: (identifier)))) right: (binary_expression left: (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (identifier)) right: (decimal_integer_literal)))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (parenthesized_expression (binary_expression left: (identifier) right: (identifier))) right: (identifier)))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (decimal_integer_literal)))))))))