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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define si(X) scanf("%d", &(X))
#define sll(X) scanf("%lld",&(X))
#define INFL 0x3f3f3f3f3f3f3f3fLL
ll mod = 1e9 + 7;
const int M = 2e6 + 5;
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll expo(ll base,ll pow){ll ans = 1;
   if(pow < 0) return 0;
   while(pow!=0){if(pow&1==1){ans = ans*base;ans = ans%mod;}
   base *= base;base%=mod;pow/=2;}return ans;}
ll inv(ll x){return expo(x,mod-2);}
ll mult_mod(ll a , ll b){a = a%mod; b = b%mod;ll c = (a * b)%mod;return c;}
ll add_mod(ll a , ll b){a = (a + mod)%mod;b = (b + mod)%mod;ll c = (a + b)%mod;return c;}
ll fac[(int)(2e6 + 7)] , inv_fac[(int)(2e6 + 7)];
void precomp_factorial(){fac[0] = 1;for(ll i = 1 ; i <= M ; i++) fac[i] = mult_mod(i , fac[i - 1]);
    inv_fac[M] = inv(fac[M]);for(ll i = M - 1 ; i >= 0 ; i--) inv_fac[i] = mult_mod(i + 1 , inv_fac[i + 1]);}
ll nCk(ll n , ll k){ll ans = fac[n];ans = mult_mod(ans , inv_fac[n - k]);ans = mult_mod(ans , inv_fac[k]);return ans;}
int dx[8] = {1 , 0 , -1 , 0 , 1 , -1 , -1 , 1};    // last 4 diagonal
int dy[8] = {0 , 1 , 0 , -1 , 1 , 1 , -1 , -1};


int main() {
    
    int n;
    cin >> n;
    cout << (n-1)/2;
}