from collections import defaultdict, deque
n, m = map(int, input().split())
graph = defaultdict(set)

for i in range(m):
    a, b = map(lambda x: int(x) - 1, input().split())
    graph[a].add(b)

INF = float("inf")
startdist = [INF for _ in range(n)]
startdist[0]=0
startD=deque()
startD.append([0,0])

while startD:
    now, dist = startD.popleft()
    if startdist[now] < dist:
        continue
    for next in graph[now]:
        if startdist[next] > dist + 1:
            startdist[next] = dist + 1
            startD.append([next, dist + 1])

if startdist[-1]==INF or startdist[-2]==INF:
    print("-1")
    exit()

INF = float("inf")
n1dist = [INF for _ in range(n)]
n1dist[-1]=0
n1D=deque()
n1D.append([n-1,0])

while n1D:
    now, dist = n1D.popleft()
    if n1dist[now] < dist:
        continue
    for next in graph[now]:
        if n1dist[next] > dist + 1:
            n1dist[next] = dist + 1
            n1D.append([next, dist + 1])



INF = float("inf")
n2dist = [INF for _ in range(n)]
n2dist[-2]=0
n2D=deque()
n2D.append([n-2,0])

while n2D:
    now, dist = n2D.popleft()
    if n2dist[now] < dist:
        continue
    for next in graph[now]:
        if n2dist[next] > dist + 1:
            n2dist[next] = dist + 1
            n2D.append([next, dist + 1])

ans = INF
ans = min(ans, startdist[-1]+n1dist[-2]+n2dist[0])
ans = min(ans, startdist[-2]+n2dist[-1]+n1dist[0])
print(ans if ans!=INF else -1)