#include <iostream>
#define llint long long

using namespace std;

llint n;
llint dp[55][2];

int main(void)
{
	cin >> n;
	
	dp[0][0] = 1;
	for(int i = 0; i < n; i++){
		if(i+1 <= n) dp[i+1][0] += dp[i][0];
		if(i+2 <= n) dp[i+2][1] += dp[i][0];
		if(i+1 <= n) dp[i+1][0] += dp[i][1];
		if(i+2 <= n) dp[i+2][1] += dp[i][1];
	}
	cout << dp[n][0] + dp[n][1] << endl;
	
	return 0;
}