local n, q = io.read("*n", "*n")
local ea, eb = {}, {}
local ccnt, parent = {}, {}
for i = 1, n do
  ccnt[i] = 0
  parent[i] = {}
end
for iq = 1, q do
  local a, b = io.read("*n", "*n")
  ea[iq], eb[iq] = a, b
  ccnt[a] = ccnt[a] + 1
  if parent[b][a] then
    parent[b][a] = parent[b][a] + 1
  else
    parent[b][a] = 1
  end
end
local tasks = {}
for i = 1, n do
  if ccnt[i] == 0 then
    table.insert(tasks, i)
  end
end
local done = 0
local function rm()
  while done < #tasks do
    done = done + 1
    local src = tasks[done]
    for p, c in pairs(parent[src]) do
      ccnt[p] = ccnt[p] - c
      if ccnt[p] == 0 then
        table.insert(tasks, p)
      end
    end
  end
end
rm()
if done == n then print(-1) os.exit() end
for iq = q, 1, -1 do
  local a, b = ea[iq], eb[iq]
  parent[b][a] = parent[b][a] - 1
  if parent[b][a] == 0 then parent[b][a] = nil end
  ccnt[a] = ccnt[a] - 1
  if ccnt[a] == 0 then
    table.insert(tasks, a)
    rm()
  end
  if done == n then print(iq) break end
end