#include <stdio.h>

long long dp[50];

long long fib(int n)
{
	if (n == 2 || n == 3) return dp[n] = n;
	if (dp[n] != -1) return dp[n];
	return dp[n] = fib(n - 1) + fib(n - 2);
}

int main()
{
	int n, i;

	scanf("%d", &n);

	for (i = 0; i < 51; i++) dp[i] = -1;

	printf("%lld\n", fib(n));

	return 0;

}