有一分数序列：2/1，3/2，5/3，8/5，13/8，21/13...求出这个数列的前n项之和,n由用户输入。（用循环语句）

numerator1 = 2
denominator1 = 1
numerator2 = 3
denominator2 = 2
sum = 2

n = int(input("请输入要求的项数："))

for i in range(3, n+1):
numerator3 = numerator1 + numerator2
denominator3 = denominator1 + denominator2
sum += numerator3/denominator3
numerator1 = numerator2
denominator1 = denominator2
numerator2 = numerator3
denominator2 = denominator3

print("前", n, "项之和为：", sum)