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This program calculates the 7th digit of a GTIN-8 barcode and calculates the validity of an 8 digit GTIN-8 barcode. Could I simplify my code any further?

import math 
def roundup(x): 
 return int(math.ceil(x / 10.0)) * 10 
def doMaths(number): 
 count = 0
 total = 0 
 for n in number: 
 n = int(n) 
 if count %2 == 0: 
 total += n * 3 
 else: 
 total += n 
 count += 1
 if len(number) == 8:
 if total %10 == 0: 
 print("Valid")
 else:
 print("Invalid")
 else:
 print(number + str(roundup(total) - total)) 
while True:
 try:
 number = input("Enter a 7 or 8 digit number for testing: ")
 i = int(number)
 if len(number) == 7 or len(number) == 8:
 doMaths(number)
 break
 else:
 print("Length of number must be 7 or 8")
 continue
 except ValueError:
 print("Only enter numbers.")
 continue