#include<cstdio>
#include<iostream>
#include <algorithm>
using namespace std;
int n;
int cnt=0;
void dfs(int cur,int res)
{
    if(res>=cur)
    {
        cnt++;
        return ;
    }
    dfs(cur,res+1);
    dfs(cur,res+2);
}
int main()
{
    cin>>n;
    dfs(n,1);
    cout<<cnt;
}