

# 백준 #5341 (Pyramids)
condition = True
while(condition) :
N = int(input())
if N == 0 :
condition = False
else :
block = 0
for i in range(N, 0, -1): # -1은 역순으로 카운트하겠다는 뜻
block = block + i
#print(i)
print(block)