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 say(obj) (cout << (obj) << endl)
#define endline (cout << endl)
#define all(container) (container).begin(), (container).end()
#define sortall(container) (sort((container).begin(), (container).end()))
#define yn(cond) ((cond) ? (cout << "yes" << endl) : (cout << "no" << endl))
#define Yn(cond) ((cond) ? (cout << "Yes" << endl) : (cout << "No" << endl))
#define YN(cond) ((cond) ? (cout << "YES" << endl) : (cout << "NO" << endl))

typedef long long       ll;

typedef vector<int>     vi;
typedef vector<ll>      vll;
typedef vector<double>  vd;
typedef vector<bool>    vb;
typedef vector<char>    vc;
typedef vector<string>  vs;
typedef pair<int, int>  pii;
typedef pair<ll, ll>    pll;
typedef vector<vi>		vvi;
typedef vector<vll>		vvll;
typedef vector<vd>		vvd;
typedef vector<vb>		vvb;
typedef vector<vc>		vvc;
typedef vector<vs>		vvs;
typedef vector<pii>     vpii;
typedef vector<pll>     vpll;

typedef struct edge_ {
	ll to, cost;
	edge_(ll to, ll cost) : to(to), cost(cost) {}
} edge;

typedef struct point_ {
	ll x, y;
	point_(ll x = 0, ll y = 0) : x(x), y(y) {}
} point;

template <typename T>
using mat2d = vector<vector<T>>;

constexpr double	PI = 3.14159265358979323846;
constexpr int		INF = (int)1e7;
constexpr ll		MOD = (ll)1e9 + 7;
constexpr double	EPS = 1e-7;
constexpr int		dy4[4] = { 0, 1, 0, -1 };
constexpr int		dx4[4] = { 1, 0, -1, 0 };
constexpr int		dy8[8] = { 0, 1, 1, 1, 0, -1, -1, -1 };
constexpr int		dx8[8] = { 1, 1, 0, -1, -1, -1, 0, 1 };

template <typename T>
T getSum(std::vector<T>& v) {
	T ret = 0;
	for (T& x : v) ret += x;
	return ret;
}

template <typename T>
istream& operator >> (istream & is, vector<T> & vec) {
	for (T& x : vec) is >> x;  return is;
}

template <typename S, typename T>
istream& operator >> (istream & is, pair<S, T> & p) {
	is >> p.first >> p.second; return is;
}

istream& operator >> (istream & is, point & p) {
	is >> p.x >> p.y;          return is;
}

template <typename T>
ostream& operator << (ostream & os, const vector<T> & vec) {
	for (const T& x : vec) os << x << " "; return os;
}

template <typename T>
ostream& operator << (ostream & os, const vector<vector<T>> & mat) {
	for (const vector<T>& m : mat)
		os << m << endl; return os;
}
template <typename S, typename T>
ostream& operator << (ostream & os, const pair<S, T> & p) {
	os << "{" << p.first << " : " << p.second << "}";
	return os;
}

template <typename T>
ostream& operator << (ostream & os, const set<T> & s) {
	for (const T& x : s) os << x << " ";
	return os;
}

ostream& operator << (ostream & os, const edge e) {
	os << "{to: " << e.to << ", cost: " << e.cost << "}";
	return os;
}

ostream& operator << (ostream & os, const point & p) {
	os << "(" << p.x << " " << p.y << ")";
	return os;
}

/******************************************************************************/
/**********************         Don't make WA !!         **********************/
/******************************************************************************/

int main()
{
	int N;
	cin >> N;
	cout << (N - 1) / 2 << endl;

	return 0;
}