A10559. 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 ( $1<=n<=10000$ ). The vertices are labeled with the numbers from 1 to $n$ . Then $n-1$ lines follow, each describing one edge using two space-separated numbers $a b$ – the labels of the vertices connected by the edge ( $1<=a<b<=n$ ). It is guaranteed that the input represents a tree.

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 $5$ .