供参考：

#include <stdio.h>
int main()
{
    int n, i = 1, k = 0, t = 1;
    double s = 0;
    scanf("%d", &n);
    while (i <= n)
    {
        k += (2 * i - 1) * t;
        s += 1.0 / k;
        t = -t;
        i++;
    }
    printf("%.4f", s);
    return 0;
}