local mmi, mma = math.min, math.max local SCCD = {} SCCD.create = function(self, n) self.n = n self.asked, self.root = {}, {} self.edge, self.edge_asked = {}, {} self.invedge, self.invedge_asked = {}, {} for i = 1, n do self.asked[i] = false self.root[i] = 0 self.edge[i] = {} self.edge_asked[i] = 0 self.invedge[i] = {} self.invedge_asked[i] = 0 end self.edgeinfo = {} end SCCD.addEdge = function(self, src, dst) table.insert(self.edge[src], dst) table.insert(self.invedge[dst], src) table.insert(self.edgeinfo, src) table.insert(self.edgeinfo, dst) end SCCD.dfs = function(self, spos, dfs_way) local asked = self.asked local edge, edge_asked = self.edge, self.edge_asked local tasks = {spos} while 0 < #tasks do local src = tasks[#tasks] asked[src] = true table.remove(tasks) if edge_asked[src] == #edge[src] then table.insert(dfs_way, src) else table.insert(tasks, src) edge_asked[src] = edge_asked[src] + 1 local dst = edge[src][edge_asked[src]] if not asked[dst] then table.insert(tasks, dst) end end end end SCCD.invdfs = function(self, spos, rootid) local asked, root = self.asked, self.root local invedge, invedge_asked = self.invedge, self.invedge_asked local tasks = {spos} while 0 < #tasks do local src = tasks[#tasks] root[src] = rootid table.remove(tasks) while invedge_asked[src] < #invedge[src] do invedge_asked[src] = invedge_asked[src] + 1 local dst = invedge[src][invedge_asked[src]] if asked[dst] and root[dst] == 0 then table.insert(tasks, src) table.insert(tasks, dst) break end end end end SCCD.categorize = function(self) local asked, root = self.asked, self.root for src = 1, self.n do if not asked[src] then local dfs_way = {} self:dfs(src, dfs_way) for i = #dfs_way, 1, -1 do local src = dfs_way[i] if root[src] == 0 then self:invdfs(src, src) end end end end end SCCD.make_group_graph = function(self) self.gsize = {} self.gedge = {} self.gpcnt = {} self.glen = {} self.gmember = {} local gsize, gmember, glen = self.gsize, self.gmember, self.glen local gedge, gpcnt = self.gedge, self.gpcnt local root = self.root local edgeinfo = self.edgeinfo for i = 1, self.n do gsize[i] = 0 gedge[i], gpcnt[i] = {}, 0 glen[i] = 1 gmember[i] = {} end for i = 1, self.n do local r = root[i] gsize[r] = gsize[r] + 1 table.insert(gmember[r], i) end for i = 1, #edgeinfo, 2 do local a, b = edgeinfo[i], edgeinfo[i + 1] local ra, rb = root[a], root[b] if ra ~= rb then glen[rb] = mma(glen[rb], glen[ra] + 1) table.insert(gedge[ra], rb) gpcnt[rb] = gpcnt[rb] + 1 end end end SCCD.toposort = function(self) self:make_group_graph() local gsize, gedge, gpcnt = self.gsize, self.gedge, self.gpcnt local topoary = {} self.topoary = topoary local topo_tasks = {} for i = 1, self.n do if 0 < gsize[i] and gpcnt[i] == 0 then table.insert(topo_tasks, i) end end local topo_done = 0 while topo_done < #topo_tasks do topo_done = topo_done + 1 local g = topo_tasks[topo_done] topoary[topo_done] = g for i = 1, #gedge[g] do local dst = gedge[g][i] gpcnt[dst] = gpcnt[dst] - 1 if gpcnt[dst] == 0 then table.insert(topo_tasks, dst) end end end return topoary end SCCD.new = function(n) local obj = {} setmetatable(obj, {__index = SCCD}) obj:create(n) return obj end local n, m = io.read("*n", "*n") local sccd = SCCD.new(n) for i = 1, m do local a, b = io.read("*n", "*n") sccd:addEdge(a, b) if a == b then print(-1) os.exit() end end sccd:categorize() local ary = sccd:toposort() local ret = 0 for i = 1, #ary do local r = ary[i] if 1 < #sccd.gmember[r] then print(-1) os.exit() end ret = mma(ret, sccd.glen[r]) end print(ret)