#include <stdio.h>
typedef unsigned long long ll;

int main() {
	int n;
	ll f[60];

	scanf("%d", &n);

	f[0] = 0; f[1] = 1; f[2] = 2;

	int i;
	for (i = 3; i <= n; i++) {
		f[i] = f[i - 1] + f[i - 2];
	}

	printf("%llu", f[n]);

	getchar(); getchar();
	return 0;
}