2021 THTB DANANG B3. Cắt dây

#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define db double
 
 
const int mm=1e6+5;
int d[100];
int main()
{
  //  freopen("corona.inp","r",stdin);
 //   freopen("corona.out","w",stdout);
    ll n;
    cin>>n;
    ll x=n/3+1,y=(n+1)/2-1;
    if((n-x)%2==1) x++;
    if((n-y)%2==1) y--;
   // cout<<x<<" "<<y<<endl;
    cout<<(y-x)/2+1;
 
}