AtCoder Regular Contest 071
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Problem Statement
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How many infinite sequences $a_1, a_2, ...$ consisting of {${1, ... ,n}$} satisfy the following conditions?
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- The
$n$-th and subsequent elements are all equal. That is, if$n \leq i,j$,$a_i = a_j$. - For every integer
$i$, the$a_i$elements immediately following the$i$-th element are all equal. That is, if$i < j < k\leq i+a_i$,$a_j = a_k$.
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Find the count modulo $10^9+7$.
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Constraints
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$1 \leq n \leq 10^6$
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Input
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Input is given from Standard Input in the following format:
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n
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Output
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Print how many sequences satisfy the conditions, modulo $10^9+7$.
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样例
Input #1
2
Output #1
4
The four sequences that satisfy the conditions are:
- $1, 1, 1, ...$
- $1, 2, 2, ...$
- $2, 1, 1, ...$
- $2, 2, 2, ...$
Input #2
654321
Output #2
968545283
题目信息
题面与样例版权归 AtCoder 及出题人所有;中文翻译由 AI 生成(CI 管线),如有出入请以原文为准。