bobo has a permutation p
1,p
2,…,p
n of 1,2,…,n.
Knowing m extra constraints of form p
ai<p
bi, bobo wanna count the number of different permutations modulo (10
9+7).
It is guaranteed that there is at least one such permutation.
The input consists of several tests. For each tests:
The first line contains n,m (1≤n≤40,0≤m≤20). Each of the following m lines contain 2 integers a
i,b
i(1≤a
i,b
i≤n).
For each tests:
A single number denotes the number of permutations.