N,M = map(int,input().split())
G = [[] for _ in range(N)]
for _ in range(M):
    u,v = map(int,input().split())
    G[u-1].append(v- 1)
    
from collections import deque

def calc(start):
    inf = 10 ** 8
    d = [inf] * N
    d[start] = 0
    stack = deque()
    
    stack.append(start)
    while stack:
        now = stack.popleft()
        for v in G[now]:
            if d[v] == inf:
                d[v] = d[now] + 1
                stack.append(v)
    return d

d0 = calc(0)
d1 = calc(N -2)
d2 = calc(N - 1)

tmp = d0[N - 2] + d2[0] + d1[-1]
tmp2 = d0[-1] + d2[N-2] + d1[0]
ans = min(tmp,tmp2)
if ans >= 10 ** 7:
    print(-1)
else:
    print(ans)