- Time limit: 1.00 s
- Memory limit: 512 MB
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.
Output
Print one integer: the number of routes modulo $10^9+7$.
Constraints
- $2 \le n \le 20$
- $1 \le m \le n^2$
- $1 \le a,b \le n$
Input:
4 6
1 2
1 3
2 3
3 2
2 4
3 4
Output:
2