#include<stdio.h>
#include<stdlib.h>


void fin(unsigned long long n){
	printf("%lld", n);
    exit(0);
}

void fib(unsigned long long n, unsigned long long a, unsigned long long b){
	if (n==0) fin(a);
	if (n==1) fin(b);
	fib(n-2, a+b, a+b+b);
}

int main(int argc, char *argv[]){
    unsigned long long n=atoll(argv[1]);
	fib(n,0,1);
}