def calculate_gold(K):
total_gold = 0
current_day = 0
current_period = 1
while current_day < K:
days_in_period = current_period
if current_day + days_in_period > K:
days_in_period = K - current_day
for _ in range(days_in_period):
total_gold += current_period
current_day += 1
current_period += 1
return total_gold
K = int(input())
print(calculate_gold(K))
