∑inai+1\sum_i^n a_i + 1∑inai+1 是 (a1+1)+(a2+1)+⋯+(an+1)(a_1 + 1) + (a_2 + 1) + \cdots + (a_n + 1)(a1+1)+(a2+1)+⋯+(an+1) 还是 a1+a2+⋯+an+1a_1 + a_2 + \cdots + a_n + 1a1+a2+⋯+an+1？
