n是人为输入，认为n大于等于3

a = 0
b = 1
sums = 0
# 无限循环判断n是否大于等于3
while True:
    n = int(input("请输入前几项和:"))
    if n >= 3:
        break
for i in range(n + 1):
    sums += a
    a, b = b, a + b
print(sums)

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