输入描述:

* Line 1: Two space-separated integers N and M

* Lines 2..1+M: Line i+1 describes the ith trail.  Each line contains
two space-separated integers u_i and v_i (1 <= u_i, v_i <= N,
u_i != v_i) describing the pair of farms connected by the
trail.

输出描述:

* Line 1: A single line containing the number of assignments of trails
to farms, taken modulo 1,000,000,007.  If no assignment
satisfies the above conditions output 0.

5 4
1 2
3 2
4 5
4 5

6

There are 6 possible assignments.  Letting {a,b,c,d} mean that farm 1
builds trail a, farm 2 builds trail b, farm 3 builds trail c, and farm 4
builds trail d, the assignments are:
{2, 3, 4, 5}
{2, 3, 5, 4}
{1, 3, 4, 5}
{1, 3, 5, 4}
{1, 2, 4, 5}
{1, 2, 5, 4}

备注:

Note that there can be two trails between the same pair of farms.