7593: [CSES Problem Set] Hamiltonian Flights
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Description
There are $n$ cities and $m$ flight connections between them.
You want to travel from Syrjälä to Lehmälä so that you visit each city exactly once.
How many possible routes are there?
You want to travel from Syrjälä to Lehmälä so that you visit each city exactly once.
How many possible routes are there?
Input
The first input line has two integers $n$ and $m$: the number of cities and flights. The cities are numbered $1,2,\dots,n$. City $1$ is Syrjälä, and city $n$ is Lehmälä.
Then, there are $m$ lines describing the flights. Each line has two integers $a$ and $b$: there is a flight from city $a$ to city $b$. All flights are one-way flights.
Then, there are $m$ lines describing the flights. Each line has two integers $a$ and $b$: there is a flight from city $a$ to city $b$. All flights are one-way flights.
Output
Print one integer: the number of routes modulo $10^9+7$.
Constraints
$2≤n≤20$
$1 \le m \le n^2$
$1 \le a,b \le n$
$1 \le m \le n^2$
$1 \le a,b \le n$
Sample 1 Input
4 6
1 2
1 3
2 3
3 2
2 4
3 4
Sample 1 Output
2