CF804A.Find Amir

A few years ago Sajjad left his school and register to another one due to security reasons. Now he wishes to find Amir, one of his schoolmates and good friends.

There are n schools numerated from 1 to n. One can travel between each pair of them, to do so, he needs to buy a ticket. The ticker between schools i and j costs and can be used multiple times. Help Sajjad to find the minimum cost he needs to pay for tickets to visit all schools. He can start and finish in any school.

几年前，萨贾德因安全原因离开了原来的学校，转到了另一所学校。现在他希望找到他的校友兼好友阿米尔。

共有 $n$ 所学校，编号从 $1$ 到 $n$。任意两所学校之间均可通行，但需购买车票。学校 $i$ 与学校 $j$ 之间的车票价格为 ，且该车票可多次使用。请帮助萨贾德计算访问所有学校的最小车票总费用。他可以在任意一所学校出发，也可在任意一所学校结束。

The first line contains a single integer n (1 ≤ n ≤ 105) — the number of schools.

第一行包含一个整数 $n$（$1 \leq n \leq 10^5$）—— 学校的数量。

Print single integer: the minimum cost of tickets needed to visit all schools.

输出一个整数：访问所有学校的车票最小总费用。

In the first example we can buy a ticket between the schools that costs .

在第一个例子中，我们可以在两所学校之间购买一张票价为 的车票。