Additive Sequence
Is the sequence additive — every term (from the third) the sum of the two before it?
Input: the first line has N; the second line has N integers.
Output: YES or NO (length ≤ 2 is trivially YES).
Constraints:
- 1 ≤ N ≤ 1000
- the values fit in 64 bits
Sample Tests
Input 1
5 1 2 3 5 8
Output 1
YES
Input 2
3 1 2 4
Output 2
NO
Discussion (opens after you solve — no spoilers before)
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