from collections import deque

INF = float("inf")
def bfs(start, edge): #始点,edge
    _N = len(edge)
    dist = [INF]*_N
    dist[start] = 0
    deq = deque()
    deq.append(start)
    while len(deq) > 0:
        crr = deq.popleft()
        for nxt in edge[crr]:
            if dist[nxt] == INF:
                dist[nxt] = dist[crr] + 1
                deq.append(nxt)
    return dist

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

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

b1 = bfs(0,E)
b2 = bfs(N-2,E)
b3 = bfs(N-1,E)

r1 = b1[N-2] + b2[N-1] + b3[0]
r2 = b1[N-1] + b3[N-2] + b2[0]
r3 = b1[N-2] + b2[0] + b1[N-1] + b1[0]

ans = min(r1,r2,r3)
if ans == INF:
    print(-1)
else:
    print(ans)