Average of Pairs

You are given an integer sequence $A$ with length $N$. Find the number of (unordered) pairs of elements such that the average of these two elements is also present in the sequence. Formally, find the number of pairs $(i,j)$ such that $1 \le i \lt j \le N$ and there is an index $k$ ($1 \le k \le N$) for which $2A_k = A_i+A_j$ holds. ### Input - The first line of the input contains a single intege