Even Pairs

Today is Aryan's Birthday. He wants an array of $N$ integers for his birthday. He had to make the array for himself as none of his friends came on his birthday. To keep himself distracted, he decided to challenge himself by solving a problem. The problem he made for himself is as follows: Aryan has to find the number of unordered pairs of indexes $i, j$ for which sum of $A_i$ and $A_j$ is even. F