Even-Sum Pairs

Difficulty: 1000 3500 ms 128 MB

math

Count the pairs i < j with a_i + a_j even.

Input: the first line has N; the second line has N integers.

Output: the number of even-sum pairs.

Constraints:

• 2 ≤ N ≤ 10⁵
• |a_i| ≤ 10⁹

Sample Tests

4
1 2 3 4

2

2
2 4

1

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