public static void main ( String [ ] args ) throws IOException {
  BufferedReader r = new BufferedReader ( new InputStreamReader ( System . in ) , 1 ) ;
  int n = Integer . parseInt ( r . readLine ( ) ) ;
  int a [ ] = new int [ 200001 ] ;
  int x [ ] = new int [ 200001 ] ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    a [ i ] = Integer . parseInt ( r . readLine ( ) ) ;
  }
  if ( a [ 0 ] > 0 ) {
    System . out . println ( - 1 ) ;
    System . exit ( 0 ) ;
  }
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    if ( a [ i + 1 ] - a [ i ] > 1 ) {
      System . out . println ( - 1 ) ;
      System . exit ( 0 ) ;
    }
  }
  long v = 0 ;
  for ( int i = 1 ;
  i <= n ;
  i ++ ) {
    if ( a [ i - 1 ] + 1 == a [ i ] ) {
      v ++ ;
    }
    else {
      v += a [ i ] ;
    }
  }
  System . out . println ( v ) ;
}

public static void main ( String [ ] args ) throws IOException { BufferedReader r = new BufferedReader ( new InputStreamReader ( System . in ) , 1 ) ; int n = Integer . parseInt ( r . readLine ( ) ) ; int a [ ] = new int [ 200001 ] ; int x [ ] = new int [ 200001 ] ; for ( int i = 0 ; i < n ; i ++ ) { a [ i ] = Integer . parseInt ( r . readLine ( ) ) ; } if ( a [ 0 ] > 0 ) { System . out . println ( - 1 ) ; System . exit ( 0 ) ; } for ( int i = 0 ; i < n ; i ++ ) { if ( a [ i + 1 ] - a [ i ] > 1 ) { System . out . println ( - 1 ) ; System . exit ( 0 ) ; } } long v = 0 ; for ( int i = 1 ; i <= n ; i ++ ) { if ( a [ i - 1 ] + 1 == a [ i ] ) { v ++ ; } else { v += a [ i ] ; } } System . out . println ( v ) ; }
(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 (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (object_creation_expression type: (type_identifier) arguments: (argument_list (field_access object: (identifier) field: (identifier)))) (decimal_integer_literal))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (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 (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))))) (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 (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))))) (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (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 (unary_expression operand: (decimal_integer_literal))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (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: (identifier)) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (array_access array: (identifier) index: (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))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (decimal_integer_literal)))))))) (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: (identifier)) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (decimal_integer_literal)) right: (array_access array: (identifier) index: (identifier)))) consequence: (block (expression_statement (update_expression (identifier)))) alternative: (block (expression_statement (assignment_expression left: (identifier) right: (array_access array: (identifier) index: (identifier)))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier))))))