CF690F1 Tree of Life (easy)

Heidi has finally found the mythical Tree of Life – a legendary combinatorial structure which is said to contain a prophecy crucially needed to defeat the undead armies. On the surface, the Tree of Life is just a regular undirected tree well-known from computer science. This means that it is a collection of $ n $ points (called vertices), some of which are connected using $ n-1 $ line segments (edges) so that each pair of vertices is connected by a path (a sequence of one or more edges). To decipher the prophecy, Heidi needs to perform a number of steps. The first is counting the number of lifelines in the tree – these are paths of length $ 2 $ , i.e., consisting of two edges. Help her!

The first line of the input contains a single integer $ n $ – the number of vertices in the tree ( $ 1

Print one integer – the number of lifelines in the tree.

In the second sample, there are four lifelines: paths between vertices $ 1 $ and $ 3 $ , $ 2 $ and $ 4 $ , $ 2 $ and $ 5 $ , and $ 4 $ and $ 5 $ .