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

for _ in range(M):
    u,v = map(int,input().split())
    G[u - 1].append(v - 1)

from collections import deque
def get_dist(start, goal, G, N):
    q = deque([])
    q.append([start, 0])
    visited = [False] * N
    visited[start] = True
    while q:
        v, d = q.popleft()
        for child in G[v]:
            if visited[child]:
                continue
            visited[child] = True
            if child == goal:
                return d + 1
            q.append([child, d + 1])
    return -1

D = [0, N - 2, N - 1]

routes = ((0,1,2,0),(0,2,1,0))
INF = 1 << 60
ans = INF
for route in routes:
    d = 0
    for i in range(len(route) - 1):
        dist = get_dist(D[route[i]], D[route[i + 1]], G, N)
        if dist == -1:
            break
        d += dist
    else:
        ans = min(ans, d)
        
if ans == INF:
    print(-1)
else:
    print(ans)