Write a Python program that inputs an integer between 0 and 1000 and adds all the digits in the integer. For example, if integer is 634, the sum of all its digits is 13.

Write a program that asks the user to enter the name of an input file. if the file does not exist, the program should prompt the user to enter the file name again. if the user types quit in any uppercase/lowercase combinations, then the program should exit without any further output.