Challenge: Arithmetic Sequence

Task

This task requires you to find the index of the first outlier in a given list. All numbers in a list are in an arithmetic sequence, except one number.

Write a program that uses a list and finds the index of the first outlier and stores it to the variable idx. The list is guaranteed to have at least $5$ elements and the first two elements will be in the required arithmetic sequence. The value of the idx should be ...