from collections import deque,defaultdict

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
 
    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]
 
    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)
 
        if x == y:
            return
 
        if self.parents[x] > self.parents[y]:
            x, y = y, x
 
        self.parents[x] += self.parents[y]
        self.parents[y] = x
 
    def size(self, x):
        return -self.parents[self.find(x)]
 
    def same(self, x, y):
        return self.find(x) == self.find(y)
 
    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]
 
    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]
 
    def group_count(self):
        return len(self.roots())
 
    def all_group_members(self):
        group_members = defaultdict(list)
        for member in range(self.n):
            group_members[self.find(member)].append(member)
        return group_members

N,M = map(int,input().split())
l = [[] for _ in range(N)]
uf = UnionFind(N)

for _ in range(M):
  a,b = map(int,input().split())
  a -= 1
  b -= 1
  l[a].append(b)
  l[b].append(a)
  uf.union(a,b)

if not uf.same(0,N-1):
  print(-1)
  exit()
uf = False
INF = 10**18
visited = [INF]*N
que = deque([(0,0)])
while que:
  i,c = que.pop()
  for j in l[i]:
    if c+1 < visited[j]:
      que.append((j,c+1))
      visited[j] = c+1
ans = visited[N-1]

print(ans)