#include<iostream>
#include<algorithm>
using namespace std;

int main(){
  int N;
  cin >> N;
  int ans;
  if (N != 1) ans = N - 1;
  else ans = 0; 
  
  cout << ans << endl;
}
#include <bits/stdc++.h> 
#include<cmath>
#include<string>
#define pb push_back
#define x first
#define y second
#define fast ios::sync_with_stdio(0),cin.tie(0),cout.tie(0)
#define all(x) x.begin(),x.end()
#define int long long
#define ll long long
using namespace std;
 
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
//void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
 
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
// look for  all edge cases
//search for a pattern

signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);	
	
	int n;
	cin >> n;
	if(n < 2){
		cout << 0;
		return 0;
	}
	
	set<pair<int,int>> s;
	for(int i=1; i<n; i++){
		int a = i;
		int b = n - i;
		s.insert({a,b});
	}
	cout << s.size();
	
	return 0;
}
