4 4
1 2
2 3
3 4
4 2

3

from collections import deque
n, m = map(int, input().split())
graph = [[] for _ in range(n+1)]
for _ in range(m):
    u, v = map(int, input().split())
    graph[u].append(v)
min_cycle = float('inf')
for start in range(1, n+1):
    dist = [-1]*(n+1)
    dist[start] = 0
    q = deque([start])
    while q:
        node = q.popleft()
        for nxt in graph[node]:
            if dist[nxt] == -1:
                dist[nxt] = dist[node]+1
                q.append(nxt)
            elif nxt == start:
                min_cycle = min(min_cycle, dist[node]+1)
print(min_cycle if min_cycle != float('inf') else -1)