N, M = map(int, input().split())
UV = [map(int, input().split()) for _ in range(M)]

graph = [[] for _ in range(N)]
for U, V in UV:
    U -= 1
    V -= 1
    graph[U].append(V)
import collections
def bfs(start, goal):
    q = collections.deque()
    q.append(start)
    reached = [0] * N
    while q:
        now = q.popleft()
        for g in graph[now]:
            if reached[g]:
                continue
            reached[g] = reached[now] + 1
            q.append(g)
    return reached[goal]

ans = 10**18
a = bfs(0, N-2)
b = bfs(N-2, N-1)
c = bfs(N-1, 0)
if a and b and c:
    ans = min(ans, a + b + c)
a = bfs(0, N-1)
b = bfs(N-1, N-2)
c = bfs(N-2, 0)
if a and b and c:
    ans = min(ans, a + b + c)

print(ans if ans != 10**18 else -1)