#solution 1068
x=int(input("enter your number:  "))
def factorial(x):
    if x>0:
        return x + factorial(x - 1)
    elif x<0:
        return x + factorial(x+1)
    if x==0:
        return 0
y=factorial(x)
if x<0:
    print(y+1)
else:
    print(y)