#include <iostream>
using namespace std;

int main()
{   int n;
	long long dp1, dp2, temp;
	cin >> n;
	dp1 = 1;
	dp2 = 2;
	for (int i = 2; i < n; i++) {
		temp = dp1 + dp2;
		dp1 = dp2;
		dp2 = temp;
	}
    cout << temp << endl;
    return 0;
}