from collections import deque

n, m = map(int, input().split())

link = {i: set() for i in range(1, n+1)}

for _ in range(m):
    a, b = map(int, input().split())
    if a != b:
        link[a].add(b)

def bfs():
    que = deque()
    que.append(1)
    visit = [0] * (n+1)
    visit[1] = 1

    step = 0
    while len(que) > 0:

        node_num = len(que)
        for _ in range(node_num):
            cur = que.popleft()
            if cur == n:
                return step

            for next in link[cur]:
                if visit[next] == 0:
                    visit[next] = 1
                    que.append(next)
        step += 1

    return -1

print(bfs())