local n, m = io.read("*n", "*n")
local edge = {}
local edge2 = {}
local edge3 = {}
for i = 1, n do edge2[i] = {} edge3[i] = {} end
for i = 1, m do
  local a, b = io.read("*n", "*n")
  table.insert(edge, {a, b})
  table.insert(edge2[a], b)
  table.insert(edge3[b], a)
end
local len = {}
local inf = 1010
for i = 1, n do len[i] = -inf end
len[1] = 0
local lim = n + 4
for i = 1, lim do
  for j = 1, m do
    local src, dst, c = edge[j][1], edge[j][2], 1
    if len[src] ~= -inf and len[dst] < len[src] + c then
      len[dst] = len[src] + c
      if i == lim then
        len[dst] = inf
      end
    end
  end
end
for i = 1, lim do
  for j = 1, m do
    local src, dst = edge[j][1], edge[j][2]
    if len[src] == inf then
      len[dst] = inf
    end
  end
end
local startpos = 0
local scand = {}
local scflag = {}
local scasked = {}
for i = 1, n do
  if len[i] == inf then
    table.insert(scand, i)
    scflag[i] = true
  else
    scflag[i] = false
  end
  scasked[i] = false
end
while 0 == startpos and 0 < #scand do
  local sc = scand[#scand]
  table.remove(scand)
  scflag[sc] = false
  scasked[sc] = true
  for i = 1, #edge3[sc] do
    local p = edge3[sc][i]
    if scasked[p] then
      startpos = p
      break
    elseif len[p] ~= inf then
      startpos = sc
      break
    else
      if not scflag[p] then
        table.insert(scand, p)
        scflag[p] = true
      end
    end
  end
end
if startpos == 0 then
  print(-1)
else
  local tasks = {startpos}
  local asked = {}
  for i = 1, n do
    asked[i] = false
  end
  local done = false
  while 0 < #tasks do
    local src = tasks[#tasks]
    table.remove(tasks)
    asked[src] = true
    while 0 < #edge2[src] do
      local dst = edge2[src][#edge2[src]]
      if dst == startpos then
        table.insert(tasks, src)
        for i = #tasks, 1, -1 do
          if tasks[i] == startpos then
            print(#tasks - i + 1)
            for j = i, #tasks do
              print(tasks[j])
            end
            break
          end
        end
        done = true
      elseif not asked[dst] then
        table.insert(tasks, src)
        table.insert(tasks, dst)
        table.remove(edge2[src])
        break
      else
        table.remove(edge2[src])
      end
      if done then break end
    end
    if done then break end
  end
end
