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: Time Limit Exceeded)

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