public static void main ( String args [ ] ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  int m = sc . nextInt ( ) ;
  int arr1 [ ] = new int [ n ] ;
  int arr2 [ ] = new int [ m ] ;
  for ( int i1 = 0 ;
  i1 < n ;
  i1 ++ ) {
    arr1 [ i1 ] = sc . nextInt ( ) ;
  }
  for ( int i2 = 0 ;
  i2 < m ;
  i2 ++ ) {
    arr2 [ i2 ] = sc . nextInt ( ) ;
  }
  Arrays . sort ( arr1 ) ;
  Arrays . sort ( arr2 ) ;
  if ( arr1 [ n - 1 ] < arr2 [ 0 ] ) {
    if ( arr1 [ 0 ] <= ( arr1 [ n - 1 ] / 2 ) ) {
      System . out . println ( arr1 [ n - 1 ] ) ;
    }
    else {
      if ( ( 2 * arr1 [ 0 ] < arr2 [ 0 ] ) ) {
        System . out . println ( 2 * arr1 [ 0 ] ) ;
      }
      else {
        System . out . println ( "-1" ) ;
      }
    }
  }
  else {
    System . out . println ( "-1" ) ;
  }
}

public static void main ( String args [ ] ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; int m = sc . nextInt ( ) ; int arr1 [ ] = new int [ n ] ; int arr2 [ ] = new int [ m ] ; for ( int i1 = 0 ; i1 < n ; i1 ++ ) { arr1 [ i1 ] = sc . nextInt ( ) ; } for ( int i2 = 0 ; i2 < m ; i2 ++ ) { arr2 [ i2 ] = sc . nextInt ( ) ; } Arrays . sort ( arr1 ) ; Arrays . sort ( arr2 ) ; if ( arr1 [ n - 1 ] < arr2 [ 0 ] ) { if ( arr1 [ 0 ] <= ( arr1 [ n - 1 ] / 2 ) ) { System . out . println ( arr1 [ n - 1 ] ) ; } else { if ( ( 2 * arr1 [ 0 ] < arr2 [ 0 ] ) ) { System . out . println ( 2 * arr1 [ 0 ] ) ; } else { System . out . println ( " - 1" ) ; } } } else { System . out . println ( " - 1" ) ; } }
(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)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (identifier))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (identifier))))) (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: (identifier)) right: (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: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))) (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (array_access array: (identifier) index: (decimal_integer_literal)))) consequence: (block (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (decimal_integer_literal)) right: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (decimal_integer_literal))))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))))))) alternative: (block (if_statement condition: (parenthesized_expression (parenthesized_expression (binary_expression left: (binary_expression left: (decimal_integer_literal) right: (array_access array: (identifier) index: (decimal_integer_literal))) right: (array_access array: (identifier) index: (decimal_integer_literal))))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (decimal_integer_literal) right: (array_access array: (identifier) index: (decimal_integer_literal))))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))))