local n, m = io.read("*n", "*n")
local len = {}
local edge = {}
for i = 1, n do
  len[i] = -1
  edge[i] = {}
end
for i = 1, m do
  local a, b = io.read("*n", "*n")
  table.insert(edge[a], b)
  table.insert(edge[b], a)
end
local tasks = {1}
len[1] = 0
local done = 0
while done < #tasks do
  done = done + 1
  local src = tasks[done]
  for i = 1, #edge[src] do
    local dst = edge[src][i]
    if len[dst] < 0 then
      len[dst] = len[src] + 1
      table.insert(tasks, dst)
    end
  end
end
print(0 < len[n] and len[n] or -1)