# DFS 解答 (想定: WA)
import sys
sys.setrecursionlimit(10**7)

INF = 1 << 60

N, M = map(int, input().split())

graph = [[] for _ in range(N)]

for _ in range(M):
    u, v = map(int, input().split())
    u -= 1; v -= 1
    graph[u].append(v)

dist = {
    0: [INF for _ in range(N)],
    N-2: [INF for _ in range(N)],
    N-1: [INF for _ in range(N)]
}

def dfs(start, now, dist):
    
    dist[start][now] = dist
    
    for next in graph[now]:
        if dist[start][next] < INF:
            continue
        dfs(start, next, dist + 1)

dfs(0, 0, 0)
dfs(N-2, N-2, 0)
dfs(N-1, N-1, 0)

ans = min(
    dist[0][N-2] + dist[N-2][N-1] + dist[N-1][0],
    dist[0][N-1] + dist[N-1][N-2] + dist[N-2][0]
)

if ans < INF:
	print(ans)
else:
	print(-1)