#include <stdio.h>

int main(){
	int n, i=1, fib1=1, fib2=1, temp;

	scanf("%d", &n);

	if(n<1){
		printf("-1\n");
		return 1;
	}

	while(i<n){
		temp = fib2;
		fib2 = fib1+fib2;
		fib1 = temp;
		i++;
	}

	printf("%d\n", fib1);

	return 0;
}