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;
using pint = pair<int, int>;
using ll = long long;
using ull = unsigned long long;
using vll = vector<long long>;
using pll = pair<ll, ll>;
#define FOR(i, begin, end) for (int i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end) for (int i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define VREP(s, ite) for (auto ite = s.begin(); ite != s.end(); ++ite)
#define FI first
#define SE second
#define ALL(v) v.begin(),v.end()
//#define endl "\n"
#define ciosup  \
    cin.tie(0); \
    ios::sync_with_stdio(false);
#define eb emplace_back
#define vint vector<int>
constexpr ll INF = 1e15 + 7LL;
constexpr ll MOD = 1e9 + 7;

template <typename T>
istream &operator>>(istream &is, vector<T> &v)
{
    for (int i = 0; i < v.size(); ++i)
    {
        is >> v[i];
    }
    return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v)
{
    for (int i = 0; i < v.size() - 1; ++i)
    {
        os << v[i] << " ";
    }
    if (v.size() > 0)
    {
        os << v[v.size() - 1] << endl;
    }
    return os;
}


int main()
{
    int n;
    cin >> n;
    ll ans = 0;
    FOR(i,1, n)
    {
        if (i > n - i)
        {
            break;
        }
        if (i != n - i)
        {
            ++ans;
        }
    }
    cout << ans << endl;
}