from collections import deque

INF = 1 << 60

N, M = map(int, input().split())

graph = [[] for _ in range(N)]

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

def bfs(start):
    distances = [INF]*N
    distances[start] = 0
    
    que = deque([(start, 0)])
    
    while que:
        now, d = que.popleft()
        
        for next in graph[now]:
            if distances[next] < INF:
                continue
            distances[next] = d + 1
            que.append((next, d + 1))
    
    return distances

dist = {
    0: bfs(0),
    N-2: bfs(N-2),
    N-1: bfs(N-1)
}

ans = min(
    dist[0][N-2] + dist[N-2][N-1] + dist[N-1][0],
    dist[0][N-1] + dist[N-1][N-2] + dist[N-2][0]
)

if ans < INF:
	print(ans)
else:
	print(-1)