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 PyPy3 (2.4.0) (judge verdict: Accepted)

#a,b = map(int,input().split())
n = int(input())
counter = 0
for a in range(1,n):
    b = n-a
    if b != a:
        #print("ab",a,b)
        counter += 1
print(counter//2)