#include <stdio.h>

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