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 Python (3.4.3) (judge verdict: Wrong Answer)

import math
N1 = input()
N = int(N1)
i = 0
j = 1
r = 0
for j in range (N):
    i=N-j
    if i!=j : 
        r+=1
        
math.floor (r/2)