#include<bits/stdc++.h>
using namespace std;
int n,i,a,b,g,j,k,h,m,c;
int factorial(int x){
return x*(x+1)/2;
}
int main()
{
cin>>n;
int f = factorial(n);
if(n>=3){
a=1,b=1;
for(i=2;i<=n;i++)
{
c=a+b;
a=b;
b=c;
}
cout<<c;
}
else if((n>0)&&(n<3))
{
j=3-n;
g=f-n+j;
cout<<g;
}
return 0;
}
