import sys

# sys.setrecursionlimit(1000005)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = -1-(-1 << 31)
# inf = -1-(-1 << 63)
# md = 10**9+7
md = 998244353

from collections import deque

def bfs(u):
    dist=[inf]*n
    dist[u]=0
    q=deque([u])
    while q:
        u=q.popleft()
        for v in to[u]:
            if dist[v]==inf:
                dist[v]=dist[u]+1
                q.append(v)
    return dist

n,m=LI()
to=[[] for _ in range(n)]
for _ in range(m):
    u,v=LI1()
    to[u].append(v)

# 0,n-2,n-1,0
# 0,n-1,n-2,0
dist=bfs(0)
d=[dist[n-2],dist[n-1]]

dist=bfs(n-2)
d[0]+=dist[n-1]
d[1]+=dist[0]

dist=bfs(n-1)
d[0]+=dist[0]
d[1]+=dist[n-2]

ans=min(d)
if ans>=inf:
    print(-1)
else:
    print(ans)