Problem: 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.

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.

Examples

Input

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


Output

3
1
2
4


Input

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


Output

-1


Input

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


Output

4
2
3
4
5

Solution (in PYTHON3):

Solution:
import sys,collections as cl,bisect as bs
sys.setrecursionlimit(100000)
input = sys.stdin.readline
mod = 10**9+7
Max = sys.maxsize
def l(): #intのlist
    return list(map(int,input().split()))
def m(): #複数文字
    return map(int,input().split())
def onem(): #Nとかの取得
    return int(input())
def s(x): #圧縮
    a = []
    if len(x) == 0:
        return []
    aa = x[0]
    su = 1
    for i in range(len(x)-1):
        if aa != x[i+1]:
            a.append([aa,su])
            aa = x[i+1]
            su = 1
        else:
            su += 1
    a.append([aa,su])
    return a
def jo(x): #listをスペースごとに分ける
    return " ".join(map(str,x))
def max2(x): #他のときもどうように作成可能
    return max(map(max,x))
def In(x,a): #aがリスト(sorted)
    k = bs.bisect_left(a,x)
    if k != len(a) and a[k] ==  x:
        return True
    else:
        return False

def pow_k(x, n):
    ans = 1
    while n:
        if n % 2:
            ans *= x
        x *= x
        n >>= 1
    return ans

"""
def nibu(x,n,r):
    ll = 0
    rr = r
    while True:
        mid = (ll+rr)//2

    if rr == mid:
        return ll
    if (ここに評価入れる):
        rr = mid
    else:
        ll = mid+1
"""

def Topo_sort(x,y):
    co = cl.Counter(n for m in x.values() for n in m)
    S = cl.deque([i for i in range(y) if co[i] == 0])
    while S:
        n = S.popleft()
        yield n
        for i in x[n]:
            co[i] -= 1
            if co[i] <= 0:
                S.append(i)
    """
    if sum(co.values()) != 0:
        print(1)
        exit()
    else:
        print(0)
        exit()
    """
    #閉路検出(1が存在,0が存在しない)
# g = cl.defaultdict(set)に有向グラフを作成し(g[~] != ~ ,yはノード数) リストが帰ってくる


    #閉路検出(1が存在,0が存在しない)
# g = cl.defaultdict(set)に有向グラフを作成し(g[~] != ~ ,yはノード数) リストが帰ってくる






n,k = m()

g = cl.defaultdict(set)

gb = cl.defaultdict(set)
for i in range(k):
    a,b = m()
    a -= 1
    b -= 1
    g[a].add(b)
    gb[b].add(a)

ans = [None for i in range(n)]

for opt in range(n):
    for start in g[opt]:
        cost = [-1]*n
        cost[start] = 0
        de = [start]
        po = []
        fl = True

        while len(de) > 0 and fl:
            po = []
            for i in de:
                for j in g[i]:
                    if cost[j] < 0:
                        cost[j] = cost[i] + 1
                        po.append(j)
                        if j == opt:
                            fl = False
            de = po

        if cost[opt] >= 0 and cost[opt] < len(ans):
            p = opt
            ans = [p+1]
            while p != start:
                for e in gb[p]:
                    if cost[p] == cost[e] + 1:
                        p = e
                        ans.append(p+1)
                        break
if ans[0] == None:
    print(-1)
else:
    print(len(ans))
    for i in ans:
        print(i)