#include <stdio.h>

int main(void){
	int n;
	scanf("%d", &n);
	long pat[n+1];
	pat[0] = 0;
	pat[1] = 1;
	pat[2] = 2;
	for(int i = 3; i <= n; i++){
		pat[i] = pat[i-2] + pat[i-1];
	}
	printf("%ld\n", pat[n]);
	return 0;
}