CF251A Points on Line

Little Petya likes points a lot. Recently his mom has presented him $ n $ points lying on the line $ OX $ . Now Petya is wondering in how many ways he can choose three distinct points so that the distance between the two farthest of them doesn't exceed $ d $ . Note that the order of the points inside the group of three chosen points doesn't matter.

The first line contains two integers: $ n $ and $ d $ ( $ 1

Print a single integer — the number of groups of three points, where the distance between two farthest points doesn't exceed $ d $ . Please do not use the %lld specifier to read or write 64-bit integers in С++. It is preferred to use the cin, cout streams or the %I64d specifier.

In the first sample any group of three points meets our conditions. In the seconds sample only 2 groups of three points meet our conditions: {-3, -2, -1} and {-2, -1, 0}. In the third sample only one group does: {1, 10, 20}.