C语言刚入门请教一下大家，这个级数求和编程哪里错了

Question

编程求以下级数前n项之和：

　s=1-1/3+1/5-1/7+1/9-1/11+1/13-1/15+.

输入：一个正整数n

输出：前n项和的值
#include<stdio.h>
#include<math.h>
int main()
{
double sum = 0;
int n;
scanf("%d",&n);
double a = 0;
for (int i = 1;i <= n;i++)
{
if (i % 2 == 0)
{

        a = a - (1 / (2 * i - 1));
        sum = sum + a;
    }
    else
    {
        a = a + (1 / (2 * i - 1));
        sum = sum + a;
    }
}
printf("%lf", sum);
return 0;
}

Answer 1

#include<stdio.h>
#include<math.h>
int main()
{
    double sum = 0;
    int n;
    scanf("%d",&n);
    double a = 0;
    for (int i = 1; i <= n; i++)
    {
        if (i % 2 == 0)
        {

            a = a - (1.0 / (2 * i - 1));

            sum = sum + a;

        }

        else

        {

            a = a + (1.0 / (2 * i - 1));

            sum = sum + a;

        }

    }

    printf("%lf", sum);

    return 0;
}

int型运算会丢失精度，改成1.0或(double)