#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
float sum1 = (1+sqrt(5)) / 2;
float sum2 = (1-sqrt(5)) / 2;
float cf(float a,int b)
{
	float sum = 1.0;
	for(int i = 1;i <= b;i++)
	{
		sum *= a;
	}
	return sum;
}
double f(int n)
{
	double sum = (cf(sum1,n)-cf(sum2,n)) / sqrt(5);
	return sum;
}
int main()
{
	int n;
	cin>>n;
	printf("%.2f",f(n));
	return 0; 
}

帮助者关注！！