public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  int [ ] arr = new int [ n ] ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) arr [ i ] = sc . nextInt ( ) ;
  int k = 0 ;
  for ( int i = 0 ;
  i < arr . length - 1 ;
  i ++ ) {
    if ( arr [ i ] <= arr [ i + 1 ] ) k ++ ;
    else break ;
  }
  if ( k == arr . length - 1 ) System . out . println ( 0 ) ;
  else {
    boolean sorted = true ;
    for ( int i = k + 1 ;
    i < arr . length - 1 ;
    i ++ ) {
      if ( arr [ i ] <= arr [ i + 1 ] ) continue ;
      else {
        sorted = false ;
        break ;
      }
    }
    if ( ! sorted ) System . out . println ( - 1 ) ;
    else if ( arr [ 0 ] >= arr [ n - 1 ] ) System . out . println ( n - k - 1 ) ;
    else System . out . println ( - 1 ) ;
  }
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; int [ ] arr = new int [ n ] ; for ( int i = 0 ; i < n ; i ++ ) arr [ i ] = sc . nextInt ( ) ; int k = 0 ; for ( int i = 0 ; i < arr . length - 1 ; i ++ ) { if ( arr [ i ] <= arr [ i + 1 ] ) k ++ ; else break ; } if ( k == arr . length - 1 ) System . out . println ( 0 ) ; else { boolean sorted = true ; for ( int i = k + 1 ; i < arr . length - 1 ; i ++ ) { if ( arr [ i ] <= arr [ i + 1 ] ) continue ; else { sorted = false ; break ; } } if ( ! sorted ) System . out . println ( - 1 ) ; else if ( arr [ 0 ] >= arr [ n - 1 ] ) System . out . println ( n - k - 1 ) ; else System . out . println ( - 1 ) ; } }
(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: (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: (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (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: (binary_expression left: (field_access object: (identifier) field: (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: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))))) consequence: (expression_statement (update_expression (identifier))) alternative: (break_statement)))) (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (binary_expression left: (field_access object: (identifier) field: (identifier)) right: (decimal_integer_literal)))) consequence: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (decimal_integer_literal)))) alternative: (block (local_variable_declaration type: (boolean_type) declarator: (variable_declarator name: (identifier) value: (true))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (identifier) right: (decimal_integer_literal)))) condition: (binary_expression left: (identifier) right: (binary_expression left: (field_access object: (identifier) field: (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: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))))) consequence: (continue_statement) alternative: (block (expression_statement (assignment_expression left: (identifier) right: (false))) (break_statement))))) (if_statement condition: (parenthesized_expression (unary_expression operand: (identifier))) consequence: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (unary_expression operand: (decimal_integer_literal))))) alternative: (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (decimal_integer_literal)) right: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))))) consequence: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (decimal_integer_literal))))) alternative: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (unary_expression operand: (decimal_integer_literal)))))))))))