#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
using namespace chrono;
#define all(a) a.begin(), a.end()
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<double> vd;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<int, ll> pil;
typedef pair<ll, int> pli;
typedef tuple<int, int, int> tpl;
template<class T>
using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<class T1, class T2>
istream &operator>>(istream &in, pair<T1, T2> &P){
	in >> P.first >> P.second;
	return in;
}
template<class T1, class T2>
ostream &operator<<(ostream &out, const pair<T1, T2> &P){
	out << "(" << P.first << ", " << P.second << ")";
	return out;
}
template<class T>
istream &operator>>(istream &in, vector<T> &arr){
	for(auto &x: arr) in >> x;
	return in;
}
template<class T>
ostream &operator<<(ostream &out, const vector<T> &arr){
	for(auto &x: arr) out << x << ' '; cout << "\n";
	return out;
}
template<class T>
istream &operator>>(istream &in, deque<T> &arr){
	for(auto &x: arr) in >> x;
	return in;
}
template<class T>
ostream &operator<<(ostream &out, const deque<T> &arr){
	for(auto &x: arr) out << x << ' '; cout << "\n";
	return out;
}
mt19937 rnd(time(0));



int main(){
	cin.tie(0)->sync_with_stdio(0);
	int n;
	cin >> n;
	cout << (n - 1) / 2;
	return 0;
}

//////////////////////////////////////////////////////////////////////////////////////////////////////
//                                                                                                  //
//                                                                                                  //
//                                              _____________                                       //
//             ++++++++++                ___------------------\                                    //
//           +++`+``+`+`++++        ///`````````````````````````````\                              //
//           ++`+`+``+++`++++  /////```````````````````````````````````\                            //
//           +++`++`+`+``+++/////`````````````````````````````````````````\                         //
//             +++`++`+``+///```````````|```````````````````````````````````\                       //
//          ____++++/++++/`````````````/````````|````````|```````````````````\                      //
//        /  /  / |    //``````````````|````````|```````|````````|````````````\                     //
//       /  /  /  | ///````````/```````|```````||```````|````````|``````\```````\                   //
//       | /  /   |///`````````|``````/````````|````````|````````|```````|```````\                  //
//       |/   |   |//``|```````|``````|````````|`````````|```````|```````|````````\                 //
//       /\___|__//`|``|```````|`    |      ``:|````````|:```````|```````|```|`````|                //
//      /     /  /``|``|``````|/     |        :|     ```:|```````|```````|``++````++                //
//     /     / //```|``|``````|      |        |:        :|    ```|```````|```++``++`\               //
//     |    /  /````|``|``````/    _.::::.    |          |      |    ````|```|`++`\`|               //
//     |   /   |````|``|`````|            `                    |       ``|```++``++`|               //
//     |  /    |````|``|`````|                                 :         |``++````++|               //
//     | /     /````|``|`````|                              _.-:::.      |..`|``.`|.|               //
//     |/     /`````|``|`````|                                     `     |```|````|`|               //
//    /|      |`````|``|`````|                    :'                    .|```|````|.|               //
//   / |      |`````|``|`````|                                         /|-|``|````|`|               //
//  /  |      |`````|```\````|                                        / ||```|````|``\              //
// /   |      |`````|````|```|::                                    /_| ||```|````|``|              //
//            |`````|````|```|:|:.        `.._                    .\___/:|```|````|``|              //
//            |`````\````|```|:|::-          ``:::....           -:|:|:::|```|````|``|              //
//            |``````|```|```|:|::`|.                          .:::|:|:::|```|````|``|              //
//             \`````|```|```|:|::/|--.                     .`:|:::|:|:::/```|````|``|              //
//              |````|```|```\:|:|:|-----             _..-:|:|:|:::|:|::|````|````|`/               //
//              |````|```|````\|:|:|-------.____.....------|/::|:::|:|::|````|````|`|               //
//              |````|```|\````\:|/\___________    ________/\--\:::|:|::|````/````|`|               //
//              |````\```| \```|:/-------------\  /----------\``\::|:|::|```/`````|`|               //
//              |`````|``|  \``|/---------------\/------------\_________|```|`````|`|               //
//                                                                                                  //
//////////////////////////////////////////////////////////////////////////////////////////////////////
//                                                                                                  //
//                                        Created by Aeren                                          //
//                                                                                                  //
//////////////////////////////////////////////////////////////////////////////////////////////////////

Measured memory_bytes: 256