from collections import defaultdict
from collections import deque
#import itertools

def bfs(start, end):
    befores = deque([-1])
    nows = deque([start])
    cnt = 0
    while True:
        if len(nows) == 0:
            return -1
        cnt += 1
        for _ in range(len(nows)):
            now = nows.popleft()
            for after in graph[now]:
                before = befores.popleft()
                if after == end:
                    return cnt
                if after != before:
                    nows.append(after)
                    befores.append(now)

N, M = map(int, input().split())
graph = defaultdict(list)
for _ in range(M):
    u, v = map(int, input().split())
    graph[u - 1].append(v - 1)
#for i in range(N):
#    print(str(i) + ": ", end = "")
#    print(*graph[i])

# route1: 0 -> N - 2 -> N - 1 -> 0
sec1 = bfs(0, N - 2)
sec2 = bfs(N - 2, N - 1)
sec3 = bfs(N - 1, 0)
route1 = sec1 + sec2 + sec3 if sec1 >= 0 and sec2 >= 0 and sec3 >= 0 else -1
# route2: 0 -> N - 1 -> N - 2 -> 0
sec1 = bfs(0, N - 1)
sec2 = bfs(N - 1, N - 2)
sec3 = bfs(N - 2, 0)
route2 = sec1 + sec2 + sec3 if sec1 >= 0 and sec2 >= 0 and sec3 >= 0 else -1
if route1 >= 0 and route2 >= 0:
    print(min(route1, route2))
elif route1 >= 0 and route2 < 0:
    print(route1)
elif route1 < 0 and route2 >= 0:
    print(route2)
else:
    print(-1)