from collections import deque

n, m = map(int, input().split())
edges = [[] for _ in range(n)]
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    edges[a].append(b)
    edges[b].append(a)
    
queue = deque()
queue.append(0)
dist = [-1] * n
dist[0] = 0
while queue:
    pos = queue.popleft()
    for npos in edges[pos]:
        if dist[npos] == -1:
            dist[npos] = dist[pos] + 1
            queue.append(npos)
print(dist[-1])