#include <bits/stdc++.h>
using namespace std;
//#include <boost/multiprecision/cpp_int.hpp>
//using multiInt = boost::multiprecision::cpp_int;

using ll = long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
template<typename Q_temp>
using smaller_queue = priority_queue <Q_temp, vector<Q_temp>, greater<Q_temp> >;

const ll MOD = (ll) 998244353;
const int INF = (int) 1e9;
const ll LINF = (ll) 4e18;
const double PI = acos(-1.0);

#define REP(i,m,n) for(ll i = m; i < (ll)(n); ++i)
#define rep(i,n) REP(i, 0, n)
#define MP make_pair
#define MT make_tuple
#define YES(n) cout << ((n) ? "YES" : "NO") << endl
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl
#define Possible(n) cout << ((n) ? "Possible" : "Impossible") << endl
#define all(v) v.begin(), v.end()
#define NP(v) next_permutation(all(v))
#define dbg(x_) cerr << #x_ << ":" << x_ << endl;
#define dbg2(x_) for(auto a_ : x_) cerr << a_ << " "; cerr << endl;
#define dbg3(x_ , sx_) rep(i, sx_) cerr << x_[i] << " "; cerr << endl;
vector<int> Dx = {0, 0, -1, 1, -1, 1, -1, 1, 0};
vector<int> Dy = {1, -1, 0, 0, -1, -1, 1, 1, 0};

inline ll CEIL(ll a, ll b) {
    return (a + b - 1) / b;
}

void finish() {
    cout << -1 << endl;
    exit(0);
}

//------------------------------------------------------

// modint (MOD で割ったあまりを扱う構造体)

template<int MOD> struct Fp {
    long long val;

    constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
        if (val < 0) v += MOD;
    }

    constexpr int getmod() {
        return MOD;
    }

    constexpr Fp operator-() const noexcept {
        return val ? MOD - val : 0;
    }

    constexpr Fp operator+(const Fp& r) const noexcept {
        return Fp(*this) += r;
    }

    constexpr Fp operator-(const Fp& r) const noexcept {
        return Fp(*this) -= r;
    }

    constexpr Fp operator*(const Fp& r) const noexcept {
        return Fp(*this) *= r;
    }

    constexpr Fp operator/(const Fp& r) const noexcept {
        return Fp(*this) /= r;
    }

    constexpr Fp& operator+=(const Fp& r) noexcept {
        val += r.val;
        if (val >= MOD) val -= MOD;
        return *this;
    }

    constexpr Fp& operator-=(const Fp& r) noexcept {
        val -= r.val;
        if (val < 0) val += MOD;
        return *this;
    }

    constexpr Fp& operator*=(const Fp& r) noexcept {
        val = val * r.val % MOD;
        return *this;
    }

    constexpr Fp& operator/=(const Fp& r) noexcept {
        long long a = r.val, b = MOD, u = 1, v = 0;
        while (b) {
            long long t = a / b;
            a -= t * b;
            swap(a, b);
            u -= t * v;
            swap(u, v);
        }
        val = val * u % MOD;
        if (val < 0) val += MOD;
        return *this;
    }

    constexpr bool operator==(const Fp& r) const noexcept {
        return this->val == r.val;
    }

    constexpr bool operator!=(const Fp& r) const noexcept {
        return this->val != r.val;
    }

    friend constexpr ostream& operator<<(ostream &os, const Fp<MOD>& x) noexcept {
        return os << x.val;
    }

    friend constexpr istream& operator>>(istream &is, Fp<MOD>& x) noexcept {
        return is >> x.val;
    }

};

Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0) return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1) t = t * a;
    return t;
}

using mint = Fp<MOD>;

// 二項係数ライブラリ

template<class T> struct BiCoef {
    vector<T> fact_, inv_, finv_;

    constexpr BiCoef() {
    }

    constexpr BiCoef(int n) noexcept : fact_(n, 1), inv_(n, 1), finv_(n, 1) {
        init(n);
    }

    constexpr void init(int n) noexcept {
        fact_.assign(n, 1), inv_.assign(n, 1), finv_.assign(n, 1);
        int MOD = fact_[0].getmod();
        for (int i = 2; i < n; i++) {
            fact_[i] = fact_[i - 1] * i;
            inv_[i] = -inv_[MOD % i] * (MOD / i);
            finv_[i] = finv_[i - 1] * inv_[i];
        }
    }

    constexpr T C(int n, int k) const noexcept {
        if (n < k || n < 0 || k < 0) return 0;
        return fact_[n] * finv_[k] * finv_[n - k];
    }

    constexpr T P(int n, int k) const noexcept {
        return C(n, k) * fact_[k];
    }

    constexpr T H(int n, int k) const noexcept {
        return C(n + k - 1, k);
    }

    constexpr T fact(int n) const noexcept {
        if (n < 0) return 0;
        return fact_[n];
    }

    constexpr T inv(int n) const noexcept {
        if (n < 0) return 0;
        return inv_[n];
    }

    constexpr T finv(int n) const noexcept {
        if (n < 0) return 0;
        return finv_[n];
    }
};

BiCoef<mint> bc;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << setprecision(20) << setiosflags(ios::fixed);

    ll n;
    cin >> n;
    cout << (n - 1) / 2 << endl;
    return 0;
}


Measured memory_bytes: 256