求交错序列前N项和

https://pintia.cn/problem-sets/13/problems/410

#include <stdio.h>
int main(void)
{
    int n;
    int numerator, denominator;
    int flag;
    double item;
    double sum;

    numerator = 1;
    denominator = 1;
    flag = 1;
    sum = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        item = flag * 1.0 * numerator / denominator;
        sum = sum + item;
        numerator = numerator + 1;
        denominator = denominator + 2;
        flag = -flag;
    }

    printf("%.3f\n", sum);

    return 0;
}