1049. 数列的片段和(20)——Python

num = input()
num = int(num)
string = input().split()
list_1 = list(string)
sum = 0
count_1 = num
count_2 = 1
length = len(list_1)
#得到结果
for i in range(length):
list_1[i] = float(list_1[i])
sum += list_1[i] * count_1 * count_2 #主要是要找到规律
count_1 -= 1
count_2 += 1
print('%.2f' %sum)