Score : $1600$ points
There is a directed graph $G$ with $N$ vertices and $M$ edges. The vertices are numbered $1$ through $N$, and the edges are numbered $1$ through $M$. Edge $i$ is directed from $x_i$ to $y_i$. Here, $x_i < y_i$ holds. Also, there are no multiple edges in $G$.
Consider selecting a subset of the set of the $M$ edges in $G$, and removing these edges from $G$ to obtain another graph $G'$. There are $2^M$ different possible graphs as $G'$.
Alice and Bob play against each other in the following game played on $G'$. First, place two pieces on vertices $1$ and $2$, one on each. Then, starting from Alice, Alice and Bob alternately perform the following operation:
The player loses when he/she becomes unable to perform the operation. We assume that both players play optimally.
Among the $2^M$ different possible graphs as $G'$, how many lead to Alice's victory? Find the count modulo $10^9+7$.
Input is given from Standard Input in the following format:
$N$ $M$ $x_1$ $y_1$ $x_2$ $y_2$ $:$ $x_M$ $y_M$
Print the number of $G'$ that lead to Alice's victory, modulo $10^9+7$.
2 1 1 2
1
The figure below shows the two possible graphs as $G'$. A graph marked with ○ leads to Alice's victory, and a graph marked with × leads to Bob's victory.
3 3 1 2 1 3 2 3
6
The figure below shows the eight possible graphs as $G'$.
4 2 1 3 2 4
2
5 10 2 4 3 4 2 5 2 3 1 2 3 5 1 3 1 5 4 5 1 4
816