from collections import defaultdict
from sys import setrecursionlimit
setrecursionlimit(10**9)

def dfs(pos,r,visited):
    global ans
    if pos in visited:
        if pos == 1 and n in visited and (n - 1) in visited:
            ans = min(ans,len(r))

        return
    
    for i in g[pos]:
        dfs(i,r+[pos], visited | set([pos]))

n,m = map(int,input().split())
g = defaultdict(list)

for i in range(m):
    u,v = map(int,input().split())
    g[u].append(v)

visited = set()
ans = 10**6
dfs(1,[],set())

if ans == 10**6:
    print(-1)
    exit()

print(ans)