from collections import deque

def bfs(s):
    dist = [inf] * n
    dist[s] = 0
    q = deque()
    q.append(s)
    while q:
        now = q.popleft()
        for nex in g[now]:
            if dist[nex] != inf:
                continue
            dist[nex] = dist[now] + 1
            q.append(nex)
    return dist

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

inf = 10 ** 18

d = {
    0 : bfs(0),
    n - 2 : bfs(n - 2),
    n - 1 : bfs(n - 1)
}

ans1 = d[0][n - 2] + d[n - 2][n - 1] + d[n - 1][0]
ans2 = d[0][n - 1] + d[n - 1][n - 2] + d[n - 2][0]

ans = min(ans1, ans2)
if ans >= inf:
    print(-1)
else:
    print(ans)