好数列
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题目背景
有一个数列，这个数列为a[],a1=1，a2=2，a3=3。a4开始，每个数为自己前面的三个数之和。
输入
输入 n。n<70
输出
输出an的值。
样例一
输入
4
输出
6
SWPU ROUND #2(DIV.3)
- Status
- Done
- Rule
- IOI
- Problem
- 6
- Start at
- 2021-11-13 9:00
- End at
- 2021-11-13 11:30
- Duration
- 2.5 hour(s)
- Host
-
mibbp
- Partic.
- 25