其实分子和分母都是斐波那契数列，满足f(n)=f(n-1)+f(n-2),很容易写出递归函数

#include<iostream>
using namespace std;
int f(int n)
{
	if (n == 1)return 1;
	if (n == 2)return 2;
	return f(n - 1) + f(n - 2);
}
int main()
{
	int n;
	cin >> n;
	double sum = 0;
	for (int i = 1; i <= n; i++)
		sum += 1.0 * f(i + 1) / f(i);
	printf("%.2f", sum);
	return 0;
}