#include <stdio.h>

int main(void){
	int i,n;
	long long int ans[60];
	
	ans[0] = 0;
	ans[1] = 1;
	ans[2] = 2;
	
	scanf("%d", &n);
	
	for(i=3;i<=n;i++){
		ans[i] = ans[i-2] + ans[i-1];
	}
	
	printf("%lld\n", ans[n]);
	return 0;
}