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, goal):
    visited = [False] * N
    que = deque([(start, 0)])
    
    while que:
        now, d = que.popleft()
        
        for next in graph[now]:
            if visited[next] == True:
                continue
            if next == goal:
                return d + 1
            visited[next] = True
            que.append((next, d + 1))
        
    return INF

# 町1 -> 町N-1 -> 町N -> 町1 の最短経路の長さ
ans_1 = bfs(0, N-2) + bfs(N-2, N-1) + bfs(N-1, 0)

# 町1 -> 町N -> 町N-1 -> 町1 の最短経路の長さ
ans_2 = bfs(0, N-1) + bfs(N-1, N-2) + bfs(N-2, 0)

ans = min(ans_1, ans_2)

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