//考虑状态机;
//开ll!
#include<bits/stdc++.h>
using namespace std;
long long n;
long long f[55][2];
int main()
{
cin >> n;
f[0][0] = 1;
for(int i = 1;i <= n;i++)
{
f[i][0] = f[i - 1][0] + f[i - 1][1];
f[i][1] = f[i - 1][0];
}
cout << f[n][0] + f[n][1] << endl;
return 0;
}