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!

海蒂终于找到了传说中的生命之树——一种著名的组合结构，据说其中蕴含着击败亡灵大军所必需的关键预言。

表面上看，生命之树只是一棵普通的无向树，这在计算机科学中早已广为人知。也就是说，它是由 $ n $ 个点（称为顶点）构成的集合，其中部分顶点通过 $ n-1 $ 条线段（边）相互连接，使得任意两个顶点之间都存在一条路径（即由一条或多条边组成的序列）。

为破译预言，海蒂需要执行若干步骤。第一步是统计树中“生命线”的数量——所谓生命线，是指长度为 2 的路径，即恰好由两条边组成的路径。请帮她完成这一任务！

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.

输入的第一行包含一个整数 $n$ —— 树中顶点的数量（$1 \leq n \leq 10000$）。顶点用 $1$ 到 $n$ 的数字标记。接下来有 $n-1$ 行，每行通过两个以空格分隔的数字 $a$ 和 $b$ 描述一条边 —— 即该边所连接的两个顶点的标签（$1 \leq a < b \leq n$）。保证输入数据表示一棵树。

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

输出一个整数——树中生命线的数量。

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

在第二个样例中，共有四条生命线：顶点 1 与 3、2 与 4、2 与 5、以及 4 与 5 之间的路径。