Problem:
Score : 100 points

Problem Statement

How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

1 \leq N \leq 10^6

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

4

Sample Output 1

1

There is only one way to choose two distinct integers totaling 4: to choose 1 and 3. (Choosing 3 and 1 is not considered different from this.)

Sample Input 2

999999

Sample Output 2

499999

Solution in Perl (v5.18.2) (judge verdict: Wrong Answer)

#A 

$in_N = <STDIN>;

$in_int_N = int($in_N);
$ans = ($in_int_N - ($in_int_N % 2)) / 2;

print "$ans\n";