另类的Sn

#include <stdio.h>
int main()
{
	int d = 1;
	int i, n;
	scanf("%d", &n);
	double sum, flag = 1;
	for(i = 1; i <= n; i++)
	{
		sum = sum + flag / d;
		d = d + 3;
		flag *= -1;
	}
	printf("sum = %.3lf", sum);
	return 0;
}