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

stack = [(0,False,False,0)]
alr = set()
for i,f1,f2,c in stack:
    if i == 0 and f1 and f2:
        print(c)
        exit()
    for j in G[i]:
        x = (j,f1|(j==N-2),f2|(j==N-1))
        if x not in alr:
            alr.add(x)
            y = (j,f1|(j==N-2),f2|(j==N-1),c+1)
            stack.append(y)
print(-1)