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)

#define MOD 998244353
#define MAX 100005
#include <stdio.h>
#include <string.h>
int nivel[MAX];
int max(int a, int b){
	return a > b ? a: b;
}
long long int ans = 1;
int main(){
	memset(nivel, 0, sizeof(int) * MAX);
	int n, maxl = 0, read;
	scanf("%d", &n);
	for(int i = 0; i < n; ++i){
		scanf("%d", &read);
		maxl = max(maxl, read);
		nivel[read]++;
	}
	for(int i = 1; i <= maxl; ++i){
		if(nivel[i] == 0){
			ans = 0;
			break;
		}
		for(int j = 0; j < nivel[i]; ++j){
		ans = (ans * nivel[i-1]);
		}
	}
	printf("%lld\n", ans % MOD);
}