#include<bits/stdc++.h>//submitRecord//
using namespace std;
//const int N=1e7+10;
int main(){
//freopen("","r",stdin);
//freopen("","w",stdout);
ios::sync_with_stdio();
cin.tie(nullptr);
cout.tie(nullptr);
long long n,a[105];
cin>>n;
a[0]=1;
a[1]=2;
a[2]=3;
a[3]=4;
for(int i=4;i<n;i++){
a[i]=a[i-1]+a[i-3];
}
cout<<a[n-1];
//fclose(stdin);
//fclose(stdout);
return 0;
}
