Costume Party

It's Halloween! Farmer John is taking his cows to a costume party, but unfortunately he only has one costume. The costume fits exactly two cows and has length $S$ ($1 \le S \le 1{,}000{,}000$). Farmer John has $N$ cows ($2 \le N \le 20{,}000$), numbered $1$ through $N$; cow $i$ has length $L_i$ ($1 \le L_i \le 1{,}000{,}000$). Two cows fit into the costume together if the sum of their lengths is at most $S$. Determine how many pairs of two distinct cows fit into the costume.

• Line 1: Two space-separated integers $N$ and $S$
• Lines 2 to $N+1$: Line $i+1$ contains a single integer $L_i$

• A single integer: the number of pairs of two distinct cows whose lengths sum to at most $S$. The order of the two cows does not matter.

In the sample, the costume length is $6$ and the cow lengths are $3, 5, 2, 1$. The four valid pairs are the cows of length $3$ and $2$; $3$ and $1$; $5$ and $1$; and $2$ and $1$.