#include <fstream>

using namespace std;

ifstream f("nunta.in");
ofstream g("nunta.out");

unsigned long long f1, f2, f3;
int n, i;

int main() {
    f >> n; f1 = 1, f2 = 2; if (n < 3) {g << (n==1?f1:f2); return 0;}
    for (i = 3; i <= n; i++) f3 = f1+f2, f1 = f2, f2 = f3;
    g << f3;
}