N, M = map(int, input().split())
adj = [[] for i in range(N+1)]
for i in range(M):
    u, v  = map(int, input().split())
    adj[u].append(v)
color = [0]*(N+1) # 0-нет цвета, 1 - обработка, 2 - отмечена и мы закончили
parent = [0]*(N+1)
for i in range(1, N+1):
    if color[i] == 0:
        stack = [i]
        while stack:
            u = stack.pop()
            if color[u] == 0:
                color[u] = 1
                stack.append(u)
                for v in adj[u]:
                    if color[v] == 0:
                        stack.append(v)
                        parent[v] = u
                    elif color[v] == 1 :
                        cycle = [v]
                        curr = u
                        while curr != v:
                            cycle.append(curr)
                            curr = parent[curr]
                        print(len(cycle))
                        print(*(cycle[::-1]))
                        exit()
            elif color[u] == 1:
                color[u] = 2

print("-1")