from collections import deque

def main():
    N,M = map(int,input().split())
    G = [[] for _ in range(N)]
    for _ in range(M):
        a,b = map(int,input().split())
        a-=1;b-=1
        G[a].append(b)
        G[b].append(a)

    Q = deque([])
    Q.append(0)
    d = [-1]*N
    d[0] = 0
    while Q:
        v = Q.popleft()
        for u in G[v]:
            if d[u] != -1: continue
            d[u] = d[v] + 1
            Q.append(u)
    print(d[N-1])

if __name__ == '__main__':
    main()