计算斐波那契数列前N项（N>=2）之和S（S=T0+T1+...Tn)的算法

def number(num):
    a=0
    b=1
    i=2
    s=1
    if num==2:
        return 1
    else:
        while 1:
            if i<num:
                a, b = b, a+b
                s = s+b
                i = i+1
            else:
                break
        return s

if __name__ == '__main__':
    num = input("请输入N")
    if num.isdigit() and int(num)>=2:
        res = number(int(num))
        print(res)
    else:
        print("输入有误")