题解 | #等差数列#
等差数列
http://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
while True:
try:
N = int(input())
a1 = 2
d = 3
sum_result = int(a1*N + 0.5*d*N*(N-1))
print(sum_result)
except:
break
