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)

import numpy as np
from scipy.sparse.csgraph import dijkstra

def get_path(start, goal, pred):
    return get_path_row(start, goal, pred[start])

def get_path_row(start, goal, pred_row):
    path = []
    i = goal
    while i != start and i >= 0:
        path.append(i)
        i = pred_row[i]
    if i < 0:
        return []
    path.append(i)
    return path[::-1]
N, M = map(int,input().split())
g = [[np.inf]*(2*N) for _ in range(2*N)]
for i in range(M):
    a,b = map(int,input().split())
    a -= 1
    b -= 1
    g[a][b] = 1
    g[a][b+N] = 1
    g[a+N][b] = 1

ans = np.inf
d = dijkstra(g,return_predecessors=True)

for i in range(N):
    if d[0][i][i+N] != np.inf:
        if ans != np.inf and len(ans)+1 <= d[0][i][i+N]:
            continue
        path = get_path(i,i+N,d[1])[::-1][:-1]
        path = [node%N for node in path]
        if len(path)==len(set(path)):
            if ans==np.inf or len(ans) > len(path):
                ans = path

if ans != INF:
    print(len(ans))
    for i in range(len(ans)):
        print(ans[i]+1)
else:
    print(ans)