#include <stdio.h>
long long int dp[51] = {0};

long long int dpfunc(int a, int n){
	long long int b = 0;
	
	if(a == n){
		return 1;
	} 
	if(dp[a+1] != 0) b += dp[a+1];
	else {
		dp[a+1] = dpfunc(a+1, n);
		b += dp[a+1];
	}
	if(a == n-1 || dp[a+2] != 0) b += dp[a+2];
	else {
		dp[a+2] = dpfunc(a+2, n);
		b += dp[a+2];
	}
	return b;
}
int main(void) {
	int n;
	scanf("%d", &n);
	printf("%lld\n", dpfunc(0, n));
	return 0;
}