题目描述
There is a sequence of length 2N: A_1, A_2, ..., A_{2N}. Each A_i is either -1 or an integer between 1 and 2N (inclusive). Any integer other than -1 appears at most once in {A_i}.
For each i such that A_i = -1, Snuke replaces A_i with an integer between 1 and 2N (inclusive), so that {A_i} will be a permutation of 1, 2, ..., 2N. Then, he finds a sequence of length N, B_1, B_2, ..., B_N, as B_i = min(A_{2i-1}, A_{2i}).
Find the number of different sequences that B_1, B_2, ..., B_N can be, modulo 10^9 + 7.
Constraints
- $1 \leq N \leq 300$
- A_i = -1 or $1 \leq A_i \leq 2N$.
- If $A_i \neq -1, A_j \neq -1$, then $A_i \neq A_j. (i \neq j)$
Input
Input is given from Standard Input in the following format:
N
A_1 A_2 ... A_{2N}
Output
Print the number of different sequences that B_1, B_2, ..., B_N can be, modulo 10^9 + 7.
Examples
Input
3
1 -1 -1 3 6 -1
Output
5
Input
4
7 1 8 3 5 2 6 4
Output
1
Input
10
7 -1 -1 -1 -1 -1 -1 6 14 12 13 -1 15 -1 -1 -1 -1 20 -1 -1
Output
9540576
Input
20
-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 6 -1 -1 -1 -1 -1 7 -1 -1 -1 -1 -1 -1 -1 -1 -1 34 -1 -1 -1 -1 31 -1 -1 -1 -1 -1 -1 -1 -1
Output
374984201
输入格式
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输出格式
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