n, m = map(int, input().split())

adj = {}
for i in range(n):
    adj[i+1] = []

for i in range(m):
    a, b = map(int, input().split())
    adj[a].append(b)
    adj[b].append(a)

visited = {}
stage = {1:0}
o=0
for i in range(10**8):
    if o:
        print(i-2)
        break
    ns = {}
    for z in stage:
        if z == n:
            o=1
            break
        if z in visited:
            pass
        visited[z] = 0
        for j in adj[z]:
            ns[j] = 0
    stage = ns