# Original solution
from collections import defaultdict
from heapq import heappop, heappush


class Graph(object):
    """
    隣接リストによる有向グラフ
    """

    def __init__(self):
        self.graph = defaultdict(list)

    def __len__(self):
        return len(self.graph)

    def add_edge(self, src, dst, weight=1):
        self.graph[src].append((dst, weight))

    def get_nodes(self):
        return list(self.graph.keys())


class Dijkstra(object):
    """
    ダイクストラ法（二分ヒープ）による最短経路探索
    計算量: O((E+V)logV)
    """

    def __init__(self, graph, start):
        self.g = graph.graph

        # startノードからの最短距離
        # startノードは0, それ以外は無限大で初期化
        self.dist = defaultdict(lambda: float('inf'))
        self.dist[start] = 0

        # 最短経路での1つ前のノード
        self.prev = defaultdict(lambda: None)

        # startノードをキューに入れる
        self.Q = []
        heappush(self.Q, (self.dist[start], start))

        while self.Q:
            # 優先度（距離）が最小であるキューを取り出す
            dist_u, u = heappop(self.Q)
            if self.dist[u] < dist_u:
                continue
            for v, weight in self.g[u]:
                alt = dist_u + weight
                if self.dist[v] > alt:
                    self.dist[v] = alt
                    self.prev[v] = u
                    heappush(self.Q, (alt, v))

    def shortest_distance(self, goal):
        """
        startノードからgoalノードまでの最短距離
        """
        return self.dist[goal]

    def shortest_path(self, goal):
        """
        startノードからgoalノードまでの最短経路
        """
        path = []
        node = goal
        while node is not None:
            path.append(node)
            node = self.prev[node]
        return path[::-1]


g = Graph()
N, M = [int(i) for i in input().split()]


for i in range(M):
    u, v = [int(i) for i in input().split()]
    g.add_edge(u-1, v-1, 1)
    g.add_edge(u-1+N, v-1, 1)
    g.add_edge(u-1, v-1+N, 1)

d = Dijkstra(g, 0)

ans_n = 10 ** 6
ans_index = -1
for i in range(N):
    d = Dijkstra(g, i)
    ans_n_ = d.dist[i+N]

    if ans_n_ < ans_n:
        ans_index = i
        ans_n = ans_n_

if ans_n == 10 ** 6:
    print((-1))
else:
    d = Dijkstra(g, ans_index)
    ans = d.shortest_path(ans_index + N)
    ans.sort()
    print(ans_n)
    for j in range(ans_n):
        print((ans[j]+1))




# Optimized solution
from collections import defaultdict
from heapq import heappop, heappush


class Graph(object):
    """
    隣接リストによる有向グラフ
    """

    def __init__(self):
        self.graph = defaultdict(list)

    def __len__(self):
        return len(self.graph)

    def add_edge(self, src, dst, weight=1):
        self.graph[src].append((dst, weight))

    def get_nodes(self):
        return list(self.graph.keys())


class Dijkstra(object):
    """
    ダイクストラ法（二分ヒープ）による最短経路探索
    計算量: O((E+V)logV)
    """

    def __init__(self, graph, start):
        self.g = graph.graph

        # startノードからの最短距離
        # startノードは0, それ以外は無限大で初期化
        self.dist = defaultdict(lambda: float('inf'))
        self.dist[start] = 0

        # 最短経路での1つ前のノード
        self.prev = defaultdict(lambda: None)

        # startノードをキューに入れる
        self.Q = []
        heappush(self.Q, (self.dist[start], start))

        while self.Q:
            # 優先度（距離）が最小であるキューを取り出す
            dist_u, u = heappop(self.Q)
            if self.dist[u] < dist_u:
                continue
            for v, weight in self.g[u]:
                alt = dist_u + weight
                if self.dist[v] > alt:
                    self.dist[v] = alt
                    self.prev[v] = u
                    heappush(self.Q, (alt, v))

    def shortest_distance(self, goal):
        """
        startノードからgoalノードまでの最短距離
        """
        return self.dist[goal]

    def shortest_path(self, goal):
        """
        startノードからgoalノードまでの最短経路
        """
        path = []
        node = goal
        while node is not None:
            path.append(node)
            node = self.prev[node]
        return path[::-1]
    

g = Graph()
N, M = [int(i) for i in input().split()]

for i in range(M):
    u, v = [int(i) for i in input().split()]
    g.add_edge(u-1, v-1, 1)
    g.add_edge(u-1, v-1+N, 1)

ans_n = 10 ** 6
ans_index = -1
for i in range(N):
    d = Dijkstra(g, i)
    ans_n_ = d.dist[i+N]

    if ans_n_ < ans_n:
        ans_index = i
        ans_n = ans_n_

if ans_n == 10 ** 6:
    print((-1))
else:
    d = Dijkstra(g, ans_index)
    ans = d.shortest_path(ans_index + N)
    ans.sort()
    print(ans_n)
    for j in range(ans_n):
        print((ans[j]+1))