from collections import deque
N,M = map(int,input().split())
G = {i:[] for i in range(1,N+1)}
for _ in range(M):
    a,b = map(int,input().split())
    G[a].append(b)
INFTY = 10**5+100
dist1 = [INFTY]*(N+1)
dist1[1] = 0
que = deque([1])
while que:
    x = que.popleft()
    for y in G[x]:
        if dist1[y]>=INFTY:
            dist1[y] = dist1[x]+1
            que.append(y)
dist2 = [INFTY]*(N+1)
dist2[N-1] = 0
que = deque([N-1])
while que:
    x = que.popleft()
    for y in G[x]:
        if dist2[y]>=INFTY:
            dist2[y] = dist2[x]+1
            que.append(y)
dist3 = [INFTY]*(N+1)
dist3[N] = 0
que = deque([N])
while que:
    x = que.popleft()
    for y in G[x]:
        if dist3[y]>=INFTY:
            dist3[y] = dist3[x]+1
            que.append(y)
ans = dist1[N-1]+dist2[N]+dist3[1]
ans = min(ans,dist1[N]+dist3[N-1]+dist2[1])
if ans>=INFTY:
    print(-1)
else:
    print(ans)