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.

Examples

Input

4


Output

1


Input

999999


Output

499999
N=input()
S=set()
for a in range(1,10**6+1):
    b=N-a
    if b>0:
        if a>b: a,b=b,a
        if a!=b:
            S.add((a,b))
    
print len(S)