Problem L. Lost In The Echo
时间限制 8000 ms
内存限制 256 MB
Charles enjoys learning. He often goes to the website Wikipedia to study computer science. Just now Charles seriously studied a series of expressions, in which algebraic expression has a great influence on him.
He is curious about how many different algebraic expressions built up from $n$ distinct variables, elementary arithmetic operations (i.e. addition, subtraction, multiplication and division), and brackets such that each variable appears exactly once and each operation is after a variable or a pair of brackets. Can you help him calculate the answer in modulo $(10^9 + 7)$?
Two algebraic expressions in this problem are considered as equivalent if and only if they can be simplified as the same rational expression. For example, assuming $a$, $b$, $c$ and $d$ are variables, $(a - d) / (b - c)$ is equivalent to $(d - a) / (c - b)$, $a / (b - c) * d$ is equivalent to $a / ((b - c) / d)$ but $a / b + c / d$ is not equivalent to $d / c + b / a$.
输入数据
输出数据
For each test case, output the answer modulo $(10^9 + 7)$ in one line.
样例输入
样例输出
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1
6
68
1170
27142
793002
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