from collections import deque
from math import inf, isinf
from typing import Iterable
import sys


def printe(*args, end="\n"):
    print(*args, end=end, file=sys.stderr)


def distance_bfs(graph: list[Iterable[int]], dep: int) -> list[int | float]:
    distance = [inf for _ in graph]
    distance[dep] = 0
    queue = deque()
    queue.append(dep)
    while queue:
        current = queue.popleft()
        for neighbor in graph[current]:
            if distance[neighbor] > distance[current] + 1:
                distance[neighbor] = distance[current] + 1
                queue.append(neighbor)
    return distance


def main():
    N, M = map(int, input().split())
    graph: list[set[int]] = [set() for _ in range(N)]
    for _ in range(M):
        dep, dest = map(lambda n: int(n) - 1, input().split())
        graph[dep].add(dest)

    from_1_distance = distance_bfs(graph, 0)
    from_N_distance = distance_bfs(graph, N - 1)
    from_N_1_distance = distance_bfs(graph, N - 2)

    # 先にN-1を通る場合
    distance_cand = from_1_distance[N - 2] + \
        from_N_1_distance[N - 1] + from_N_distance[0]
    # 先にNを通る場合
    distance_cand2 = from_1_distance[N - 1] + \
        from_N_distance[N - 2] + from_N_1_distance[0]
    min_distance = min(distance_cand, distance_cand2)
    if isinf(min_distance):
        print(-1)
    else:
        print(min_distance)


if __name__ == "__main__":
    main()