line = input()
try:
    n = int(line)
except Exception as e:
    print("Hello World")
sum = 0
cnt = 0
try:
    while True:
        s = int(input())
        sum += s
except Exception as e:
    if cnt == 0:
        print(int(n * (n + 1) / 2))
    else:
        print(int(s))