#include <bits/stdc++.h>
    #include <random>

    using namespace std;

    typedef unsigned long long ull;
    typedef long long ll;
    typedef double ld;
    //#define ll __int128
    //#define int ll
    //#define int ll
    //#define char ll
    #define double ld
    typedef vector<char> vc;
    typedef vector<vc> vvc;
    typedef vector<vvc> vvvc;
    typedef pair<int, int> pii;
    typedef pair<pii, pii> piii;
    typedef pair<ll, ll> pll;
    typedef vector<int> vi;
    typedef vector<pii> vpi;
    typedef vector<vi> vvi;
    typedef vector<vvi> vvvi;
    typedef vector<short> vs;
    typedef vector<vs> vvs;
    typedef vector<vvs> vvvs;
    typedef vector<ll> vl;
    typedef vector<vl> vvl;
    typedef vector<vvl> vvvl;
    typedef vector<ld> vld;
    typedef vector<vld> vvld;
    typedef vector<vvld> vvvld;
    typedef vector<string> vst;
    typedef vector<vst> vvst;
    typedef pair<ld, ld> pld;

    #define inmin(a, b) a = min(a, (b))
    #define inmax(a, b) a = max(a, (b))
    #define ALL(a) a.begin(),a.end()
    #define RALL(a) a.rbegin(),a.rend()
    #define sqr(x) ((x) * (x))
    #define fori(i, n) for(int i = 0; i < int(n); ++i)
    #define SZ(a) ((int)((a).size()))
    #define triple(T) tuple<T, T, T>
    #define quad(T) tuple<T, T, T, T>
    #define watch(x) cerr << (#x) << " = " << (x) << endl;

    #ifdef MAX_HOME
    #define cerr cout
    #else
    #define cerr if (false) cerr
    #endif

    const double PI = 2 * acos(0.0);
    #define rand shittttty_shit
    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
    mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());

    const string DIGITS = "0123456789";
    const string ALPH = "abcdefghijklmnopqrstuvwxyz";


    template<class T0, class T1>
    inline ostream &operator<<(ostream &out, pair<T0, T1> &a) {
        return out << "{" << a.first << ", " << a.second << "}";
    }

    template<class T0, class T1>
    inline istream &operator>>(istream &in, pair<T0, T1> &a) {
        return in >> a.first >> a.second;
    }

    template<class T0, class T1, class T2>
    inline ostream &operator<<(ostream &out, tuple<T0, T1, T2> &a) {
        return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << "}";
    }

    template<class T0, class T1, class T2, class T3>
    inline ostream &operator<<(ostream &out, tuple<T0, T1, T2, T3> &a) {
        return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << ", " << get<3>(a) << "}";
    }

    template<class T>
    inline ostream &operator<<(ostream &out, vector<T> &a) {
        out << "[";
        fori (i, a.size())out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
        return out;
    }


    void smain();


    signed main() {
        ios::sync_with_stdio(0);
        cin.tie(0);
        cout.tie(0);
    #ifdef MAX_HOME
        freopen("input.txt", "r", stdin);
        clock_t start = clock();
    #endif
        cout << setprecision(12) << fixed;
        smain();
    #ifdef MAX_HOME
        cout << "\n\nTOTAL EXECUTION TIME: " << float(clock() - start) / CLOCKS_PER_SEC << endl;
    #endif
        return 0;
    }

    void smain() {
        ll n;
        cin >> n;
        cout << (n - 1) / 2;
    }

Measured memory_bytes: 3664