Distinct Pairs Sum

Given an integer N, your task is to count the number of distinct pairs of positive integers (a, b) such that a + b = N. Note that the pair (a, b) is considered the same as (b, a), so each pair is counted only once.

If N < 3, there are no such pairs since both a and b must be at least 1. For N ≥ 3, the number of distinct pairs is given by the formula:

$$\left\lfloor \frac{N-1}{2} \right\rfloor$$

Your program should read the input from stdin and output the answer to stdout.

inputFormat

The input consists of a single integer N (1 ≤ N ≤ 10⁹) provided via standard input.

outputFormat

Output a single integer representing the number of distinct pairs (a, b) such that a + b = N.

## sample

5

2

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