编写代码求分数序列： 求出这个数列2/1，-3/2，5/3，-13/5，-21/13的前20项之和

numerator = [2, -3]
denominator = [1, 2]

for i in range(18):
new_numerator = -numerator[-1] - 2 * denominator[-1]
new_denominator = numerator[-1]
numerator.append(new_numerator)
denominator.append(new_denominator)

result = sum([numerator[i] / denominator[i] for i in range(20)])
print(result) # 输出结果为：-3.3955001588272854