1040  Costume Party
Time Limit : 1 Second
Memory Limit : 128 MB
Submission: 67
Solved: 24
 Description
 It's Halloween! Farmer John is taking the cows to a costume party,
but unfortunately he only has one costume. The costume fits precisely
two cows with a length of S (1 <= S <= 1,000,000). FJ has N cows
(2 <= N <= 20,000) conveniently numbered 1..N; cow i has length L_i
(1 <= L_i <= 1,000,000). Two cows can fit into the costume if the
sum of their lengths is no greater than the length of the costume.
FJ wants to know how many pairs of two distinct cows will fit into
the costume.
 Input
 * Line 1: Two spaceseparated integers: N and S
* Lines 2..N+1: Line i+1 contains a single integer: L_i
 Output
 * Line 1: A single integer representing the number of pairs of cows FJ
can choose. Note that the order of the two cows does not
matter.
 sample input

4 6 3 5 2 1
 sample output

4
 hint
 source
 USACO JAN08