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

def dfs(pos,step,cnt):
    global ans
    if pos == 1 and cnt == 2:
        ans = min(ans,step)
        return
    if pos in visited: return
    
    visited.add(pos)
    
    for i in g[pos]:
        if pos == n or pos == n-1:
            dfs(i,step+1,cnt+1)
        else:
            dfs(i,step+1,cnt)

        visited.discard(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(1,0,0)

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

print(ans)