#include <stdio.h>

int main(){
	int fn1=1, fn2=1, tmp, n;

	scanf("%d", &n);
	if(n<=0){ //brojimo od 1, mada moze i od 0
		printf("-1\n");
		return 1;
	}

	while(n>1){
		tmp = fn2;
		fn2 = fn1 + fn2;
		fn1 = tmp;
		n--;
	}

	printf("%d\n", fn1);

	return 0;
}