题解 | 等差数列
等差数列
https://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
while True:
try:
n = int(input())
sum = 0
for i in range(1,n+1):
sum += 3*(i-1)
print(sum+ n*2)
except:
break
