Arithmetic Progression Checker

You are given a sequence of integers. The first integer n represents the number of elements in the sequence. The following n integers form the sequence. Your task is to determine if the sequence forms an arithmetic progression. An arithmetic progression is a sequence in which the difference between any two consecutive numbers is the same. If the sequence is an arithmetic progression, output the common difference; otherwise output NO.

Note: All formulae must be rendered in LaTeX. For example, the common difference is defined as \(d = a_2 - a_1\), and for all \(i\) from 2 to \(n\), \(a_i - a_{i-1} = d\).

inputFormat

The input is given as a single line via standard input. The first integer represents \(n\), the number of elements in the sequence, followed by \(n\) space-separated integers representing the sequence.

outputFormat

If the sequence is an arithmetic progression, print the common difference; otherwise, print NO. The output should be written to standard output.

4 2 4 6 8

2