Write a complete C++ program that does the following: (a) It asks the user to enter a positive integer. (b) The program reads a value x entered by the user. If the value is not legal, the program repeatedly makes the user type another value until a legal value of x has been entered. (c) The program prints a triangular display which has the number x on its top row. Each later row is obtained by omitting the last digit from the number on the previous row. For example, if the user entered 29763 for x, the program prints the following picture. 29763 2976 297 29