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 C (GCC 5.4.1) (judge verdict: Wrong Answer)

int search(int k,int l,int r){
  if(r>=l){
  int mid = l+(r-l)/2;
   if(mid == k) return 1;
    if(mid > k)
      return search(k,l+1,mid);
    else
      return search(k,mid+1,r);
  }
  return 0;
}

int main(){
  int n,i,j,c=0;
  scanf("%d",&n);
  for(i=1;i<=n/2;i++){
    if(search(n-i,i,n) == 1)c++;
  }
  printf("%d\n",c);
  return 0;
}