求画一个NS流程图！！急！

#include <stdio.h>
int main()
{
    int a = 1, b = 1, flag = 1;
    double sum = 0;
    int n;
    scanf_s("%d", &n);
    for (int i = 1; i <= n; i++)
    {
        sum += a * 1.0 * flag / b;
        flag++;
        a = -a;
        b += 2;
    }
    printf("%.3lf", sum);
    return 0;
}

简单的思路