You are given a list of n-1 integers and these integers are in the range of 1 to n. There are no duplicates in the list. One of the integers is missing in the list. Write an efficient code to find the missing integer.

Example:

Input: arr[] = {1, 2, 4, 6, 3, 7, 8}

Output: 5

Explanation: The missing number from 1 to 8 is 5