#include <bits/stdc++.h>
using namespace std;
// tourist's modular-arithmetic class
template <typename T>
T inverse(T a, T m) {
    T u = 0, v = 1;
    while (a != 0) {
        T t = m / a;
        m -= t * a; swap(a, m);
        u -= t * v; swap(u, v);
    }
    assert(m == 1);
    return u;
}

template <typename T>
class Modular {
public:
    using Type = typename decay<decltype(T::value)>::type;

    constexpr Modular() : value() {}
    template <typename U>
    Modular(const U& x) {
        value = normalize(x);
    }

    template <typename U>
    static Type normalize(const U& x) {
        Type v;
        if (-mod() <= x && x < mod()) v = static_cast<Type>(x);
        else v = static_cast<Type>(x % mod());
        if (v < 0) v += mod();
        return v;
    }

    const Type& operator()() const { return value; }
    template <typename U>
    explicit operator U() const { return static_cast<U>(value); }
    constexpr static Type mod() { return T::value; }

    Modular& operator+=(const Modular& other) { if ((value += other.value) >= mod()) value -= mod(); return *this; }
    Modular& operator-=(const Modular& other) { if ((value -= other.value) < 0) value += mod(); return *this; }
    template <typename U> Modular& operator+=(const U& other) { return *this += Modular(other); }
    template <typename U> Modular& operator-=(const U& other) { return *this -= Modular(other); }
    Modular& operator++() { return *this += 1; }
    Modular& operator--() { return *this -= 1; }
    Modular operator++(int) { Modular result(*this); *this += 1; return result; }
    Modular operator--(int) { Modular result(*this); *this -= 1; return result; }
    Modular operator-() const { return Modular(-value); }

    template <typename U = T>
    typename enable_if<is_same<typename Modular<U>::Type, int>::value, Modular>::type& operator*=(const Modular& rhs) {
#ifdef _WIN32
        uint64_t x = static_cast<int64_t>(value) * static_cast<int64_t>(rhs.value);
        uint32_t xh = static_cast<uint32_t>(x >> 32), xl = static_cast<uint32_t>(x), d, m;
        asm(
        "divl %4; \n\t"
        : "=a" (d), "=d" (m)
        : "d" (xh), "a" (xl), "r" (mod())
        );
        value = m;
#else
        value = normalize(static_cast<int64_t>(value) * static_cast<int64_t>(rhs.value));
#endif
        return *this;
    }
    template <typename U = T>
    typename enable_if<is_same<typename Modular<U>::Type, int64_t>::value, Modular>::type& operator*=(const Modular& rhs) {
        int64_t q = static_cast<int64_t>(static_cast<long double>(value) * rhs.value / mod());
        value = normalize(value * rhs.value - q * mod());
        return *this;
    }
    template <typename U = T>
    typename enable_if<!is_integral<typename Modular<U>::Type>::value, Modular>::type& operator*=(const Modular& rhs) {
        value = normalize(value * rhs.value);
        return *this;
    }

    Modular& operator/=(const Modular& other) { return *this *= Modular(inverse(other.value, mod())); }

    template <typename U>
    friend const Modular<U>& abs(const Modular<U>& v) { return v; }

    template <typename U>
    friend bool operator==(const Modular<U>& lhs, const Modular<U>& rhs);

    template <typename U>
    friend bool operator<(const Modular<U>& lhs, const Modular<U>& rhs);

    template <typename U>
    friend std::istream& operator>>(std::istream& stream, Modular<U>& number);

private:
    Type value;
};


template <typename T> bool operator==(const Modular<T>& lhs, const Modular<T>& rhs) { return lhs.value == rhs.value; }
template <typename T, typename U> bool operator==(const Modular<T>& lhs, U rhs) { return lhs == Modular<T>(rhs); }
template <typename T, typename U> bool operator==(U lhs, const Modular<T>& rhs) { return Modular<T>(lhs) == rhs; }

template <typename T> bool operator!=(const Modular<T>& lhs, const Modular<T>& rhs) { return !(lhs == rhs); }
template <typename T, typename U> bool operator!=(const Modular<T>& lhs, U rhs) { return !(lhs == rhs); }
template <typename T, typename U> bool operator!=(U lhs, const Modular<T>& rhs) { return !(lhs == rhs); }

template <typename T> bool operator<(const Modular<T>& lhs, const Modular<T>& rhs) { return lhs.value < rhs.value; }

template <typename T> Modular<T> operator+(const Modular<T>& lhs, const Modular<T>& rhs) { return Modular<T>(lhs) += rhs; }
template <typename T, typename U> Modular<T> operator+(const Modular<T>& lhs, U rhs) { return Modular<T>(lhs) += rhs; }
template <typename T, typename U> Modular<T> operator+(U lhs, const Modular<T>& rhs) { return Modular<T>(lhs) += rhs; }

template <typename T> Modular<T> operator-(const Modular<T>& lhs, const Modular<T>& rhs) { return Modular<T>(lhs) -= rhs; }
template <typename T, typename U> Modular<T> operator-(const Modular<T>& lhs, U rhs) { return Modular<T>(lhs) -= rhs; }
template <typename T, typename U> Modular<T> operator-(U lhs, const Modular<T>& rhs) { return Modular<T>(lhs) -= rhs; }

template <typename T> Modular<T> operator*(const Modular<T>& lhs, const Modular<T>& rhs) { return Modular<T>(lhs) *= rhs; }
template <typename T, typename U> Modular<T> operator*(const Modular<T>& lhs, U rhs) { return Modular<T>(lhs) *= rhs; }
template <typename T, typename U> Modular<T> operator*(U lhs, const Modular<T>& rhs) { return Modular<T>(lhs) *= rhs; }

template <typename T> Modular<T> operator/(const Modular<T>& lhs, const Modular<T>& rhs) { return Modular<T>(lhs) /= rhs; }
template <typename T, typename U> Modular<T> operator/(const Modular<T>& lhs, U rhs) { return Modular<T>(lhs) /= rhs; }
template <typename T, typename U> Modular<T> operator/(U lhs, const Modular<T>& rhs) { return Modular<T>(lhs) /= rhs; }

template<typename T, typename U>
Modular<T> power(const Modular<T>& a, const U& b) {
    assert(b >= 0);
    Modular<T> x = a, res = 1;
    U p = b;
    while (p > 0) {
        if (p & 1) res *= x;
        x *= x;
        p >>= 1;
    }
    return res;
}

template <typename T>
bool IsZero(const Modular<T>& number) {
    return number() == 0;
}

template <typename T>
string to_string(const Modular<T>& number) {
    return to_string(number());
}

template <typename T>
std::ostream& operator<<(std::ostream& stream, const Modular<T>& number) {
    return stream << number();
}

template <typename T>
std::istream& operator>>(std::istream& stream, Modular<T>& number) {
    typename common_type<typename Modular<T>::Type, int64_t>::type x;
    stream >> x;
    number.value = Modular<T>::normalize(x);
    return stream;
}

struct fast_ios {
    fast_ios() {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(10);
    };
} fast_ios_;

#define LSON(x) ((x) * 2)
#define RSON(x) ((x) * 2 + 1)
template<typename A, typename B>
void Min(A &a, const B &b){
    if (b < a) a = b;
}
template<typename A, typename B>
void Max(A &a, const B &b){
    if (b > a) a = b;
}

int cas;
ostream& kase() {
    return cout << "Case #" << ++cas << ": ";
}

#if __cplusplus < 201402L
template<class Iterator>
std::reverse_iterator<Iterator> make_reverse_iterator(Iterator it)
{
    return std::reverse_iterator<Iterator>(it);
}
#endif

template <typename iter_t>
struct iter_pair {
    iter_t _beg, _end;
    iter_t begin(){return _beg;}
    iter_t end(){return _end;}
};

template<class cont> iter_pair<reverse_iterator<decltype(begin(declval<cont>()))>>
reverse(cont &&r) {
    return {make_reverse_iterator(end(r)), make_reverse_iterator(begin(r))};
}

template<typename T> void dprintln(const T &t) { cout << t << endl; } // for debug use
template<typename T, typename ...Args> void dprintln(const T &t, const Args &...rest) { cout << t << ' '; dprintln(rest...); }
template<typename T> void println(const T &t) { cout << t << '\n'; }


template<typename T, typename ...Args> void println(const T &t, const Args &...rest) { cout << t << ' '; println(rest...); }
template<typename T> void println(const vector<T>& vec) {
    if (!vec.empty()) {
        cout << vec[0];
        for (size_t i = 1; i < vec.size(); ++i)
            cout << ' ' << vec[i];
    }
    cout << '\n';
}

template<typename T> void print(const T &t) { cout << t << ' '; }

template<typename T, typename ...Args> void print(const T &t, const Args &...rest) { cout << t << ' '; print(rest...); }

// this overload is chosen when there's only one argument
template<class T> void scan(T &t) { cin >> t; }
template<class T, class ...Args> void scan(T &a, Args &...rest) { cin >> a; scan(rest...); }
template<typename T> istream& operator>>(istream& stream, vector<T>& vec) {
    for (auto& x : vec) stream >> x;
    return stream;
}
using ull = unsigned long long;
using ll = long long;
using ul = unsigned long;
using vl = vector<ll>;
using vi = vector<int>;
using pii = pair<int,int>;
using pip = pair<int,pii>;
using pll = pair<ll,ll>;
using vb = vector<bool>;
using vpii = vector<pii>;
using ldb = long double;
template <typename int_t>
inline int_t lowbit(int_t x) {return x & -x;}
#define rng(i, a, b) for(int i = (int)(a); i < (int)(b); ++i)
#define up(i, a, b) for (int i = int(a); i <= int(b); ++i)
#define down(i, b, a) for (int i = int(b); i >= int(a); i--)
#define rep(n) for(int _iter_##n = 0, _num_##n = (int)n; _iter_##n < _num_##n; ++_iter_##n)
#define stp(i, a, b, c) for (int i = (a); i < (b); i += (c))
#define FOR(x, cont) for (const auto &x: cont)
#define INC(init, x, y) for (init; x <= y; ++x)
#define For(x, cont) for (auto &x: cont)
#define all(x) begin(x), end(x)
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define SZ(x) (int)(x).size()
#define UP(i, l, r) for(i = decltype(i)(l); i <= decltype(i)(r); ++i)
#define DOWN(i, r, l) for (i = decltype(i)(r); i >= decltype(i)(l); --i)
#define Dec(a, b) for (; a >= b; --a)
template <typename T, typename Comp = less<T>>
using pq = priority_queue<T, vector<T>, Comp>;
#define popcnt(x) __builtin_popcountll((x))
#define SET(arr, v) memset(arr, (v), sizeof (arr))
#define UNIQ(vec) (vec).erase(unique(all(vec)), end(vec))
#define LB(cont, x) int(lower_bound(all(cont), x) - begin(cont))
#define UB(cont, x) int(upper_bound(all(cont), x) - begin(cont))
#define AI(name, n, m) vv<int> name(n, vi(m));
#define AL(name, n, m) vv<ll> name(size_t(n), vl(size_t(m)));
#define set0(arr) memset(arr, 0, sizeof arr)
#define set1(arr) memset(arr, -1, sizeof arr)
#define AT(T, n, m, a)  vector<vector<T>> a(n, vector<T>(m))
const int dx[] = {0, 0, -1, 1}, dy[] = {-1, 1, 0, 0};
auto bet = [](const ll x, const ll y, const ll i) {
    return x <= i && i <= y;
};

template<typename T1, typename T2>
T1 ceil(T1 x, T2 y) { // y >= 1，是整数。需要注意 x + y - 1 是否会溢出
    return (x + y - 1) / y;
}

inline int h_bit(unsigned long long x) {
    return (int)sizeof(unsigned long long) * 8 - 1 - __builtin_clzll(x);
}

int pow2(int x){ // power of 2
    return int(1U << h_bit((ull)x)) + (x != lowbit(x));
}

template <typename T>
struct BIT {
    vector<T> a;
    function<T(T,T)> bin_op;
    const T init;
    explicit BIT(int n, function<T(T,T)> bin_op, T init=T()):bin_op(bin_op), init(init) {
        a.assign(n + 1, init);
    }

    T prefix(int x) {
        auto res = init;
        while (x) {
            res = bin_op(a[x], res);
            x -= x & -x;
        }
        return res;
    }
    void modify(int x, T v) {
        while (x < (int)a.size()) {
            a[x] = bin_op(a[x], v);
            x += x & -x;
        }
    }
    void clear(){
        fill(a.begin(), a.end(), init);
    }
};

vi get_prime(int n) {
    vi minp((ul) n + 1), p;
    for (int i = 2; i <= n; i++) {
        if (!minp[i]) {
            minp[i] = i;
            p.pb(i);
        }
        FOR(x, p) {
            if (x <= minp[i] && x * i <= n)
                minp[x * i] = x;
            else break;
        }
    }
    return p;
}

// alias templates
template<typename T> using vv = vector<vector<T>>;
template <typename T1, typename T2 = T1> using vp = vector<pair<T1,T2>>;
template<typename T, int n> using va = vector<array<T,n>>;

//order_of_key (k) : Number of items strictly smaller than k .
//find_by_order(k) : K-th element in a set (counting from zero).
#ifdef __GNUC__
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
template <typename T>
using rank_tree = __gnu_pbds::tree<
        T,
        __gnu_pbds::null_type,
        less<T>,
        __gnu_pbds::rb_tree_tag,
        __gnu_pbds::tree_order_statistics_node_update>;
#endif

//并查集
struct UnionFind {
    vi par, sz;
    int n_tree;
    explicit UnionFind(int n) { //0-indexed
        par.assign(n, 0);
        sz.assign(n, 1);
        rng (i, 0, n) par[i] = i;
        n_tree = n;
    }
    int n_cluster() const {
        return n_tree;
    }
    int size(int x) {
        return sz[root(x)];
    }
    int root(int x) {
        return x == par[x] ? x : par[x] = root(par[x]);
    }
    bool unite(int x, int y) {
        int rx = root(x), ry = root(y);
        if(rx != ry) {
            par[rx] = ry;
            --n_tree;
            sz[ry] += sz[rx];
            return true;
        }
        return false;
    }
};

template<typename T, typename Compare = std::less<T>>
struct SparseTable {
    size_t n{}; // 0-indexed
    vv<T> a;

    template<typename ptr_t>
    SparseTable(ptr_t beg, ptr_t end):n(end - beg) {
        a.resize((size_t) h_bit(n) + 1); // 注意：不能写成 h_bit(n)
        a[0].assign(beg, end);
        rng (i, 0, SZ(a) - 1) {
            a[i + 1].resize(n);
            rng(j, 0, n - (1u << i)) {
                a[i + 1][j] = min(a[i][j], a[i][j + (1u << i)], Compare());
            }
            rng(j, n - (1u << i), n) {
                a[i + 1][j] = a[i][j];
            }
        }
    }
    using idx_t = long;
    T query(idx_t l, idx_t r) { // l <=  r
        int i = h_bit(r - l + 1ul);
        return min(a[i][l], a[i][r + 1 - (1u << i)], Compare());
    }
};

vi get_popcnt(int n) {
    vi res((ul)n + 1);
    rng (i, 0, n) {
        if (i * 2 <= n) res[i * 2] = res[i];
        if ((i & 1) == 0) res[i + 1] = res[i] + 1;
    }
    return res;
}

vi get_mu(int n) {
    assert(n > 0);
    vi mu(n + 1);
    vi min_p(n + 1);
    vi prime;
    mu[1] = 1;
    rng (i, 2, n + 1) {
        if (!min_p[i]) {
            prime.pb(i);
            min_p[i] = i;
            mu[i] = -1;
        }
        FOR (p, prime) {
            if (p > min_p[i]) {
                break;
            }
            int t = p * i;
            if (t > n) break;
            min_p[t] = p;
            mu[t] = p == min_p[i] ? 0 : -mu[i];
        }
    }
    return mu;
}

template<typename num>
num fp(num x, ll n, int mod) { //fast power: hat off to quailty
    if (n < 0) {
        x = inverse(x, mod);
        n = -n;
    }
    num ans = 1;
    while (n) {
        if (n & 1) ans *= x;
        n /= 2;
        x *= x;
    }
    return ans;
}

// debug code by tourist
string to_string(const string& s) {
    return '"' + s + '"';
}

string to_string(const char* s) {
    return to_string((string) s);
}

string to_string(bool b) {
    return (b ? "true" : "false");
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
    return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A>
string to_string(A v) {
    bool first = true;
    string res = "{";
    for (const auto &x : v) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(x);
    }
    res += "}";
    return res;
}

void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << " " << to_string(H);
    debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif
// end DEBUG

template <typename T>
struct Binom {
    vector<T> fact, inv_fact;

    explicit Binom(int n) : fact(n + 1), inv_fact(n + 1) {
        fact[0] = 1;
        up (i, 1, n) fact[i] = fact[i - 1] * i;
        inv_fact[n] = 1 / fact[n];
        down (i, n, 1) {
            inv_fact[i - 1] = inv_fact[i] * i;
        }
    }

    T get_binom(int x, int y) const {
        assert(x <= SZ(fact) - 1);
        assert(x >= 0 && y >= 0);
        if (x < y) return 0;
        return fact[x] * inv_fact[y] * inv_fact[x - y];
    }

    T get_fact(int n) const {
        assert(n < SZ(fact));
        return fact[n];
    }
};

//https://codeforces.com/blog/entry/22072
struct HLD {
    HLD(int n, const int *heavy_son, const int *fa, const int *depth) : top(n + 1), pos(n + 1), depth(depth), fa(fa) {
        int index = 0;
        // 节点编号从1开始
        for (int i = 1; i <= n; i++) {
            if (i != heavy_son[fa[i]]) {
                for (int j = i; j != 0; j = heavy_son[j]) {
                    top[j] = i;
                    pos[j] = ++index; // pos从1开始
                }
            }
        }
    }

    template<typename BinOpr>
    //返回值是 LCA(u,v)
    int process_path(int u, int v, BinOpr op, bool value_on_edge = false) const {
        while (top[u] != top[v]) {
            if (depth[top[u]] < depth[top[v]]) swap(u, v);
            op(pos[top[u]], pos[u]);
            u = fa[top[u]];
        }
        if (depth[u] > depth[v]) swap(u, v);
        op(pos[u] + value_on_edge, pos[v]);
        return u;
    }

    vector<int> top, pos;
    const int *depth, *fa;
};

template <typename T>
T get_mid(T l, T r) {
    assert(l <= r);
    return l + (r - l) / 2;
}
template <typename T>
int get_bit(T a, int i) {
    return a >> i & 1;
}
/*
using ModType = int;

struct VarMod { static ModType value; };
ModType VarMod::value;
ModType& md = VarMod::value;
using Mint = Modular<VarMod>;
 */
constexpr int md = 20050321;
using Mint = Modular<std::integral_constant<decay<decltype(md)>::type, md>>;
//////////////////^"^///////////////////////////////////


template <typename Arc>
struct SCC {
    int n;
    using Graph = vector<vector<Arc>>;
    const Graph &g;
    function<int(const Arc &)> get_v;
    vector<int> low, order, scc_id;
    vector<bool> in_stack;
    int time, scc_cnt;
    stack<int> s;

    SCC(int n, const Graph &g, function<int(const Arc &e)> get_v = [](const Arc &e) { return e; }) :
            n(n), g(g), get_v(move(get_v)), low(n + 1), order(n + 1), scc_id(n + 1, -1), in_stack(n + 1), time(0),
            scc_cnt(0)
            {} // SCCs are indexed from 0
    void dfs(int u) {
//        debug(u);
        order[u] = low[u] = ++time;
        s.push(u);
        in_stack[u] = true;

        for (const auto &e : g[u]) {
            int v = get_v(e);  // 酌情修改
            if (!order[v]) {
                dfs(v);
                low[u] = min(low[u], low[v]);
            } else if (in_stack[v]) {
                low[u] = min(low[u], order[v]);
            }
        }

        if (order[u] == low[u]) {
            while (true) {
                int v = s.top();
                s.pop();
                in_stack[v] = false;
                scc_id[v] = scc_cnt;
                if (u == v) break;
            }
            ++scc_cnt;
        }
    }

    int find_scc() {
        for (int i = 1; i <= n; i++) {
            if (!order[i]) {
                dfs(i);
            }
        }
        return scc_cnt;
    }

    int find_scc_from(int start) {
        dfs(start);
        return scc_cnt;
    }
};

int main() {
#ifdef LOCAL
    ifstream in("main.in");
    cin.rdbuf(in.rdbuf());
//    ofstream out("main.out");
//    cout.rdbuf(out.rdbuf());
#endif

int n;
scan(n);

println(n/2 - (n%2==0));






#ifdef LOCAL
    cout << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
#endif

    return 0;
}



Measured memory_bytes: 256