c++求數列3： 1+1/（1-3）+1/（1-3+5）+...+1/(1-3+5-...+2n-1)的值

#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;
int main()
{
	int n;
	double fac(double);
	cin>>n;
	double sum=0.0;
	for(double i=1;i<=n;i++)
	{
		sum=sum+1/fac(i);
	}
	cout<<fixed<<setprecision(3)<<sum<<endl;
	return 0;
 } 
 double fac(double m)
 {
 	float sum1=0.0;
 	for(int i=1;i<=m;i++)
 	{
 		if(i%2==0)
 		sum1=sum1-(2*i-1);
 		else sum1=sum1+(2*i-1);
	 }
	 return sum1;
 }