find missing number in an array in python

1def get_missing_summatin(a):
2  n = a[-1]
3  total = n*(n+1)//2
4  summation = 0
5  summation = sum(a)
6  result = total-summation
7  print(result)
8
9a = [1,2,3,5,6,7]
10get_missing_summatin(a)