import java.io.BufferedReader import java.io.InputStream import java.io.InputStreamReader import java.lang.AssertionError import java.util.* import kotlin.math.abs import kotlin.math.max import kotlin.math.min val MOD = 998244353 class UnionFind(n: Int) { private val par = IntArray(n){it} private val rank = IntArray(n){1} // 集合の要素数 private val visits = IntArray(n) // 訪れた場所をfind毎に用意するのがもったいないのでつかいまわす fun find(x: Int): Int { var ptr = 0 var i = x while(par[i] != i) { visits[ptr++] = i i = par[i] } for (j in 0 until ptr) { par[visits[j]] = i } return i } private fun merge(node: Int, rt: Int): Int { par[node] = rt rank[rt] += rank[node] return rt } fun unite(x: Int, y: Int): Boolean { val x1 = find(x) val y1 = find(y) return if (x1 == y1) false else { if (rank[x1] < rank[y1]) merge(x1, y1) else merge(y1, x1) true } } fun isSame(x: Int, y: Int) = find(x) == find(y) fun isRoot(x: Int) = par[x] == x /** * xを解決する必要がないときは直にrankをみる */ fun cntNodes(x: Int): Int = rank[find(x)] fun inspect() = run{par} } /** * listをgroupに分けたいときに、Array(N){mutableList()}が重いので、軽い実装 * * @param N 要素のサイズ * @param M グループのサイズ */ class GroupedArray(val N: Int, val M: Int) { private val rt = IntArray(M){-1} private val next = IntArray(N){-2} private val cnt = IntArray(M) fun setGroup(i: Int, g: Int) { next[i] = rt[g] rt[g] = i cnt[g]++ } fun elements(g: Int): IntArray? { if (cnt[g] == 0) return null val res = IntArray(cnt[g]) var cur = rt[g] for (i in 0 until cnt[g]) { res[i] = cur cur = next[cur] } return res } } /** * 連結成分毎にsubgraphに分けて、subgraph上で番号を振り直す処理をまとめたもの */ class GroupedByDSU(private val N: Int, private val M: Int, private val from: IntArray, private val to: IntArray) { data class Group(val N: Int, val from: IntArray, val to: IntArray) private val mapping = IntArray(N) // zip_id -> node_id private val inv = IntArray(N) // node_id -> zip_id private val grp = IntArray(N) private val grpNodes = GroupedArray(N, N) private val grpEdges = GroupedArray(M, N) fun build() { val uf = UnionFind(N) for (i in 0 until M) { uf.unite(from[i], to[i]) } for (i in 0 until N) { grp[i] = uf.find(i) grpNodes.setGroup(i, grp[i]) } for (i in 0 until M) { val g = grp[from[i]] grpEdges.setGroup(i, g) } } /** * @return (grp, zip_id -> node_id のmapping) */ fun get(g: Int): Pair? { val nodes = grpNodes.elements(g) ?: return null val edges = grpEdges.elements(g) ?: return null for (i in nodes.indices) { val v = nodes[i] mapping[i] = v inv[v] = i } val gFrom = IntArray(edges.size) val gTo = IntArray(edges.size) for (i in edges.indices) { gFrom[i] = inv[from[edges[i]]] gTo[i] = inv[to[edges[i]]] } val group = Group(nodes.size, gFrom, gTo) return Pair(group, mapping) } } fun packDGraph(n: Int, from: IntArray, to: IntArray): Array { val p = IntArray(n) val m = from.size for (i in 0 until m) { ++p[from[i]] } val g = Array(n){IntArray(p[it])} for (i in 0 until m) { g[from[i]][--p[from[i]]] = to[i] } return g } /** * @return node->order じゃなくて、nodeのリストを返す。 */ fun topologicalSort(n: Int, g: Array): IntArray? { val res = IntArray(n){-1} var ptr = 0 val que = ArrayDeque() val deg = IntArray(n) for (v in 0 until n) { for (o in g[v]) { deg[o]++ } } for (v in 0 until n) { if (deg[v] == 0) que.add(v) } while(que.isNotEmpty()) { val v = que.poll() res[ptr++] = v for (i in g[v].indices) { val o = g[v][i] if (--deg[o] == 0) que.add(o) } } // 全部に順序がつかないとループがあったってこと // グラフにでてこないノードにも順序がつく return if (ptr < n) null else res } class Solver(stream: InputStream, private val out: java.io.PrintWriter) { private val reader = BufferedReader(InputStreamReader(stream), 32768) fun solve() { val (N, M) = na(2) val (from, to) = na2(M, -1) val ans = mutableListOf>() val grouped = GroupedByDSU(N, M, from, to) grouped.build() for (gId in 0 until N) { val (grp, map) = grouped.get(gId) ?: continue val n = grp.N val g = packDGraph(n, grp.from, grp.to) val top = topologicalSort(n, g) if (top == null) { // 一回転 for (i in 0 until n) { ans += Pair(map[i], map[(i + 1)%n]) } } else { // 一直線 for (o in 0 until n - 1) { ans += Pair(map[top[o]], map[top[o + 1]]) } } } out.println(ans.size) for (i in ans) { out.println("${i.first+1} ${i.second+1}") } } private val isDebug = try { // なんか本番でエラーでる System.getenv("MY_DEBUG") != null } catch (t: Throwable) { false } private var tokenizer: StringTokenizer? = null private fun next(): String { while (tokenizer == null || !tokenizer!!.hasMoreTokens()) { tokenizer = StringTokenizer(reader.readLine()) } return tokenizer!!.nextToken() } private fun ni() = next().toInt() private fun nl() = next().toLong() private fun ns() = next() private fun na(n: Int, offset: Int = 0): IntArray { return IntArray(n) { ni() + offset } } private fun nal(n: Int, offset: Int = 0): LongArray { val res = LongArray(n) for (i in 0 until n) { res[i] = nl() + offset } return res } private fun na2(n: Int, offset: Int = 0): Array { val a = Array(2){IntArray(n)} for (i in 0 until n) { for (e in a) { e[i] = ni() + offset } } return a } private inline fun debug(msg: () -> String) { if (isDebug) System.err.println(msg()) } /** * コーナーケースでエラー出たりするので、debug(dp[1])のように添え字付きの場合はdebug{}をつかうこと */ private inline fun debug(a: LongArray) { debug { a.joinToString(" ") } } private inline fun debug(a: IntArray) { debug { a.joinToString(" ") } } private inline fun debug(a: BooleanArray) { debug { toString(a) } } private inline fun toString(a: BooleanArray) = run{a.map { if (it) 1 else 0 }.joinToString("")} private inline fun debugDim(A: Array) { if (isDebug) { for (a in A) { debug(a) } } } private inline fun debugDim(A: Array) { if (isDebug) { for (a in A) { debug(a) } } } private inline fun debugDim(A: Array) { if (isDebug) { for (a in A) { debug(a) } } } /** * 勝手にimport消されるのを防ぎたい */ private fun hoge() { min(1, 2) max(1, 2) abs(-10) } private inline fun assert(b: Boolean) = run{if (!b) throw AssertionError()} private inline fun assert(b: Boolean, f: () -> String) = run{if (!b) throw AssertionError(f())} } fun main() { val out = java.io.PrintWriter(System.out) Solver(System.`in`, out).solve() out.flush() }