The Missing Term

Difficulty: 1200 3000 ms 128 MB

math

An arithmetic progression had its terms listed in order, but one interior term was deleted. Recover it.

Input: the first line has the remaining count M; the second line has the M remaining terms in order.

Output: the deleted term.

Constraints:

• 3 ≤ M ≤ 10⁵
• the common difference is non-zero; the deleted term is not an endpoint

Sample Tests

4
1 3 7 9

5

3
10 20 40

30

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