How do I alternate the sign of a summation series

I am trying to code the summation of the following series:

I can’t figure out how to alternate the sign from "-" to "+" to "-" and so on….

Here is my code:

#include<stdio.h>

int main()
{
    int i=1,n;
    float sum,num,den;
    den=2;
    sum=0;

    printf("Enter number of terms for summation: ");
    scanf("%d",&n);

    for(i=1;i<=n;i++)
    {
        num=i*(i+1);
        den=den*(i+2);
        sum+=num/den;
    }
    printf("%f",sum);
}

>Solution :

int main()
{
    int i=1,n, sign = 1;
    float sum,num,den;
    den=2;
    sum=0;

    printf("Enter number of terms for summation: ");
    scanf("%d",&n);

    for(i=1;i<=n;i++)
    {
        num=i*(i+1);
        den=den*(i+2);
        sum+=sign * num/den;
        sign *= -1;
    }
    printf("%f",sum);
}