While loop 86 sum_func

def sum_func(a):
    if type(a) is int:
        if a < 0:
            return -1
        else:
            sum = 0
            integer = 0
            while integer <= a:
                sum = sum + integer
                integer = integer + 1
            return sum
    else:
        return -1

why are we taking while loop and integer,sum ??