#include <iostream>
#include <iomanip>
#include <stdlib.h>
#include <stdio.h>
#include <algorithm>
#include <assert.h>
#include <bitset>
#include <cstring>
#include <cstdlib>
#include <ctime>
#include <map>
#include <math.h>
#include <malloc.h>
#include <numeric>
#include <string>
#include <stack>
#include <queue>
#include <vector>
#pragma warning(disable:4996)
#define REP(variable, repeatnumber) for(int variable=0; variable<(repeatnumber); ++variable)
#define FOR(variable, start, end) for(int variable=(start); variable<=(end); ++variable)
#define RFOR(variable, start, end) for(int variable=(start); variable>=(end); --variable)
#define ULL unsigned long long
#define LL long long
using namespace std; // 700B
LL a = 0, b = 0, c = 0, d = 0, n;
int t;
int main(){
cin >> n;
if (!(n & 1))
n -= 1;
else
n = (n - 1) / 2;
cout << n << endl;
return 0;
}
