def f(x):
    if x==1:
        return 1
    elif x==2:
        return 2
    else:
        return f(x-1)+f(x-2)
n=int(input())
print(f(n))

#include<bits/stdc++.h>
using namespace std;
int main()
{
	int a[10086]={0};
	a[1]=1;
	a[2]=2;
	int n;
	cin>>n;
	for(int i=3;i<=n;i++)
	{
		a[i]=a[i-1]+a[i-2];
	}
	cout<<a[n];
}

C++ :

#include <iostream>
using namespace std;
int a[1000]={0};
int main()
{
    int n;
    cin>>n;
    int a=1,b=2,c=3,d=3;
    if(n==1){cout<<a;return 0;}
    if(n==2){cout<<b;return 0;}
    if(n==3){cout<<c;return 0;}
    for(int i=4; i<=n; i++){
        c=b+c;
        a=b;
        b=d;
        d=b+c;
    }
    cout<<c<<endl;
    return 0;
}