Arithmetic Progression Subsequences (2) X55634

Write a program that reads an integer n>1 followed by a sequence of integers, and finds out whether the sequence contains a consecutive subsequence of length n that forms an arithmetic progression.

A consecutive subsequence of integers forms an arithmetic progression if the difference between two consecutive numbers equals a fixed integer value r. For instance, 4 5 6 7 is an arithmetic progression with r = 1, and 22 33 44 55 66 is an arithmetic progression with r = 11.

If the input sequence contains such a progression, the program must report the start number and the value r. Otherwise, the program must indicate "No arithmetic progression of length n found".

Input The input is an integer n>1, followed by a sequence of integers containing at least 2 elements.

Output If a progression subsequence of length n exists, the output is the first element of the subsequence and the value of r. Otherwise, the output is "No arithmetic progression of length n found".