#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);(i)++)

i64 N;
int main(){
  cin >> N;
  if(N == 0 && N == 1){
    cout << 0 << endl;
    return 0;
  }
  cout << N - 1 - (N & 1 ? 2 : 0) << endl;
}