Problem:
Score : 600 points

Problem Statement

Given is a directed graph G with N vertices and M edges.

The vertices are numbered 1 to N, and the i-th edge is directed from Vertex A_i to Vertex B_i.

It is guaranteed that the graph contains no self-loops or multiple edges.

Determine whether there exists an induced subgraph (see Notes) of G such that the in-degree and out-degree of every vertex are both 1. If the answer is yes, show one such subgraph.

Here the null graph is not considered as a subgraph.

Notes

For a directed graph G = (V, E), we call a directed graph G' = (V', E') satisfying the following conditions an induced subgraph of G:

V' is a (non-empty) subset of V.

E' is the set of all the edges in E that have both endpoints in V'.

Constraints

1 \leq N \leq 1000

0 \leq M \leq 2000

1 \leq A_i,B_i \leq N

A_i \neq B_i

All pairs (A_i, B_i) are distinct.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
:
A_M B_M

Output

If there is no induced subgraph of G that satisfies the condition, print -1.
Otherwise, print an induced subgraph of G that satisfies the condition, in the following format:

K
v_1
v_2
:
v_K

This represents the induced subgraph of G with K vertices whose vertex set is \{v_1, v_2, \ldots, v_K\}. (The order of v_1, v_2, \ldots, v_K does not matter.)
If there are multiple subgraphs of G that satisfy the condition, printing any of them is accepted.

Sample Input 1

4 5
1 2
2 3
2 4
4 1
4 3

Sample Output 1

3
1
2
4

The induced subgraph of G whose vertex set is \{1, 2, 4\} has the edge set \{(1, 2), (2, 4), (4, 1)\}. The in-degree and out-degree of every vertex in this graph are both 1.

Sample Input 2

4 5
1 2
2 3
2 4
1 4
4 3

Sample Output 2

-1

There is no induced subgraph of G that satisfies the condition.

Sample Input 3

6 9
1 2
2 3
3 4
4 5
5 6
5 1
5 2
6 1
6 2

Sample Output 3

4
2
3
4
5

Solution in Python (3.4.3) (judge verdict: Runtime Error)

from collections import deque
import sys
input=sys.stdin.readline
n,m=map(int,input().split())
ab=[list(map(int,input().split())) for i in range(m)]
g=[[] for i in range(n+1)]
for a,b in ab:
  g[a].append(b)
st=0
for i in range(1,n+1):
  vis=[0]*(n+1)
  q=deque([i])
  while q:
    x=q.popleft()
    if vis[x]==1:
      st=x
      break
    vis[x]=1
    for y in g[x]:
      q.append(y)
  if st:
    break
if st==0:
  print(-1)
  exit()
q=deque([st])
route=[[] for i in range(n+1)]
ans = 0
while q:
  x=q.popleft()
  for y in g[x]:
    route[y].extend(route[x])
    route[y].append(x)
    q.append(y)
    if route[y][0]==y:
      ans=route[y]
      break
  if ans:
    break
print(len(ans))
print(*ans,sep="\n")