while True:

    try:

        n = int(input())

        area = 2

        for i in range(n-1):

            area += i+2

        print(area)

    except:

        break
while True:

    try:

        n = int(input())

        print(1 + n*(n+1)/2)

    except:

        break
while True:
    try:
        n = int(input())
        area = 2
        for i in range(n - 1):
            area += i + 2
        print(area)
    except:
        break

while True:
    try:
        n = int(input())
        print(1 + n * (n + 1) / 2)
    except:
        break

-        area = 2 -        for i in range(n - 1): -            area += i + 2 -        print(area) +        print(1 + n * (n + 1) / 2)