from collections import deque

N,M=map(int,input().split())
E=[[] for i in range(N+1)]

for i in range(M):
    x,y=map(int,input().split())
    E[x].append(y)

def distance_from(x):
    DIS=[1<<30]*(N+1)
    DIS[x]=0

    Q=deque([x])

    while Q:
        x=Q.popleft()

        for to in E[x]:
            if DIS[to]>DIS[x]+1:
                DIS[to]=DIS[x]+1
                Q.append(to)

    return DIS

ONE=distance_from(1)
DN=distance_from(N-1)
DN1=distance_from(N)

ANS=ONE[N]+DN1[N-1]+DN[1]
ANS2=ONE[N-1]+DN[N]+DN1[1]

LA=min(ANS,ANS2)

if LA>10**7:
    print(-1)
else:
    print(LA)