#!/usr/bin/env python3
import sys

def main():
    N, M = map(int, input().split())
    INF = 10 ** 16
    from collections import deque
    def bfs(edges: "List[to]", start_node: int) -> list:
        q = deque()
        dist = [INF] * len(edges)
        q.append(start_node)
        dist[start_node] = 0
        while q:
            now = q.popleft()
            for next in edges[now]:
                if dist[next] != INF:
                    continue
                q.append(next)
                dist[next] = dist[now] + 1
        return dist
    G = [[] for _ in range(N)]
    for _ in range(M):
        a, b = map(lambda i: int(i) - 1, input().split())
        G[a].append(b)
        G[b].append(a)
    d = bfs(G, 0)
    ans = d[N - 1]
    print(-1 if ans == INF else ans)



if __name__ == '__main__':
    main()