如何做到一个分数序列和

#include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
int main()
{
    int n;
    double sum=0;
    cin>>n;
    for(int i=1;i<=n;++i)
    {
        if(i%2==0)
        {
            sum-=1/(double)(i);
        }
        else
        {
            sum+=1/(double)(i);
        }
    }
    printf("%.4lf",sum);
}