Challenge: Arithmetic Sequence

Task

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

Write a program that uses an array and finds the index of the first outlier and stores it to the variable idx. The array 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 $-1$ ...