Problem:
Score : 700 points

Problem Statement

You are given a simple connected undirected graph with N vertices and M edges. The vertices are numbered 1 to N, and the i-th edge connects Vertex A_i and Vertex B_i.
Takahashi will assign one of the two possible directions to each of the edges in the graph to make a directed graph.
Determine if it is possible to make a directed graph with an even number of edges going out from every vertex. If the answer is yes, construct one such graph.

Notes

An undirected graph is said to be simple when it contains no self-loops or multiple edges.

Constraints

2 \leq N \leq 10^5

N-1 \leq M \leq 10^5

1 \leq A_i,B_i \leq N (1\leq i\leq M)

The given graph is simple and connected.

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
:
A_M B_M

Output

If it is impossible to assign directions to satisfy the requirement, print -1.
Otherwise, print an assignment of directions that satisfies the requirement, in the following format:

C_1 D_1
:
C_M D_M

Here each pair (C_i, D_i) means that there is an edge directed from Vertex C_i to Vertex D_i. The edges may be printed in any order.

Sample Input 1

4 4
1 2
2 3
3 4
4 1

Sample Output 1

1 2
1 4
3 2
3 4

After this assignment of directions, Vertex 1 and 3 will each have two outgoing edges, and Vertex 2 and 4 will each have zero outgoing edges.

Sample Input 2

5 5
1 2
2 3
3 4
2 5
4 5

Sample Output 2

-1

Solution in Scala (2.11.7) (judge verdict: Runtime Error)

object Main {
  import java.io.{BufferedReader, InputStream, InputStreamReader}
  import java.util.StringTokenizer
  import scala.reflect.ClassTag

  def main(args: Array[String]): Unit = {
    val out = new java.io.PrintWriter(System.out)
    new Main(out, new InputReader(System.in)).solve()
    out.flush()
  }

  private val oj = System.getenv("ATCODER_DEBUG") == null
  def DEBUG(f: => Unit): Unit = {
    if (!oj){ f }
  }
  def debug(as: Array[Boolean]): Unit = if (!oj){ debug(as.map(x => if(x) "1" else "0").mkString) }
  def debug(as: Array[Int]): Unit = if (!oj){ debug(as.mkString(" ")) }
  def debug(as: Array[Long]): Unit =if (!oj){ debug(as.mkString(" ")) }
  def debugDim(m: Array[Array[Long]]): Unit = if (!oj){
    REP(m.length) { i =>
      debug(m(i))
    }
  }
  def debugDimFlip(m: Array[Array[Long]]): Unit = if (!oj){
    REP(m(0).length) { j =>
      REP(m.length) { i =>
        System.err.print(m(i)(j))
        System.err.print(" ")
      }
      System.err.println()
    }
  }
  def debug(s: => String): Unit = DEBUG {
    System.err.println(s)
  }
  def debugL(num: => Long): Unit = DEBUG {
    System.err.println(num)
  }

  class InputReader(val stream: InputStream) {
    private val reader = new BufferedReader(new InputStreamReader(stream), 32768)
    private var tokenizer: StringTokenizer = _

    def next(): String = {
      while (tokenizer == null || !tokenizer.hasMoreTokens)
        tokenizer = new StringTokenizer(reader.readLine)
      tokenizer.nextToken
    }

    def nextInt(): Int = Integer.parseInt(next())
    def nextLong(): Long = java.lang.Long.parseLong(next())
    def nextChar(): Char = next().charAt(0)

    def ni(): Int = nextInt()
    def nl(): Long = nextLong()
    def nc(): Char = nextChar()
    def ns(): String = next()
    def ns(n: Int): Array[Char] = ns().toCharArray
    def na(n: Int, offset: Int = 0): Array[Int] = map(n)(_ => ni() + offset)
    def na2(n: Int, offset: Int = 0): (Array[Int], Array[Int]) = {
      val A1, A2 = Array.ofDim[Int](n)
      REP(n) { i =>
        A1(i) = ni() + offset
        A2(i) = ni() + offset
      }
      (A1, A2)
    }
    def nm(n: Int, m: Int): Array[Array[Int]] = {
      val A = Array.ofDim[Int](n, m)
      REP(n) { i =>
        REP(m) { j =>
          A(i)(j) = ni()
        }
      }
      A
    }
    def nal(n: Int): Array[Long] = map(n)(_ => nl())
    def nm_c(n: Int, m: Int): Array[Array[Char]] = map(n) (_ => ns(m))
  }

  def REP(n: Int, offset: Int = 0)(f: Int => Unit): Unit = {
    var i = offset
    val N = n + offset
    while(i < N) { f(i); i += 1 }
  }
  def REP_r(n: Int, offset: Int = 0)(f: Int => Unit): Unit = {
    var i = n - 1 + offset
    while(i >= offset) { f(i); i -= 1 }
  }
  def TO(from: Int, to: Int)(f: Int => Unit): Unit = {
    REP(to - from + 1, from) { i =>
      f(i)
    }
  }
  def map[@specialized A: ClassTag](n: Int, offset: Int = 0)(f: Int => A): Array[A] = {
    val res = Array.ofDim[A](n)
    REP(n)(i => res(i) = f(i + offset))
    res
  }

  def sumL(as: Array[Int]): Long = {
    var s = 0L
    REP(as.length)(i => s += as(i))
    s
  }
  def cumSum(as: Array[Int]): Array[Long] = {
    val cum = Array.ofDim[Long](as.length + 1)
    REP(as.length) { i =>
      cum(i + 1) = cum(i) + as(i)
    }
    cum
  }
}

class Main(out: java.io.PrintWriter, sc: Main.InputReader) {
  import sc._
  import Main._
  import java.util.Arrays.sort

  import scala.collection.mutable
  import math.{abs, max, min}
  import mutable.ArrayBuffer

  // toIntとか+7とかするならvalにしろ
  @inline private def MOD = 1000000007

  case class Edge(i: Int, a: Int, b: Int) {
    def oppo(v: Int) = if (a == v) b else a
  }
  case class Entry(v: Int, deg: Int) extends Comparable[Entry] {
    override def compareTo(o: Entry): Int = Integer.compare(deg, o.deg)
  }

  def solve2(): Boolean = {
    val N, M = ni()
    if (M % 2 == 1) return false

    val deg = Array.ofDim[Int](N)
    val E = Array.ofDim[Edge](M)
    val g = Array.fill[mutable.Set[Int]](N)(mutable.Set()) //edge番号
    REP(M) { i =>
      val a, b = ni() - 1
      E(i) = Edge(i, a, b)
      deg(a) += 1
      deg(b) += 1
      g(a) += i
      g(b) += i
    }

    val q = new java.util.PriorityQueue[Entry]
    def addEntry(v: Int): Unit = {
      if (deg(v) > 0) q.add(Entry(v, deg(v)))
    }

    debug(deg)

    REP(N) { i =>
      addEntry(i)
    }

    val ans = ArrayBuffer[(Int, Int)]()

    // v -> e1, v -> e2
    def outward2(v: Int, e1: Edge): Unit = {
      g(v).remove(e1.i)
      g(E(e1.i).oppo(v)).remove(e1.i)
      val e2 = E(g(v).head)
      g(v).remove(e2.i)
      g(E(e2.i).oppo(v)).remove(e2.i)

      debug(s"outward2($v) e1:$e1 e2:$e2")

      deg(v) -= 2
      deg(e1.oppo(v)) -= 1
      deg(e2.oppo(v)) -= 1

      addEntry(v)
      addEntry(e1.oppo(v))
      addEntry(e2.oppo(v))

      ans += v -> e1.oppo(v)
      ans += v -> e2.oppo(v)
    }

    // e1 -> v -> e2
//    def passOver(v: Int): Unit = {
//      val e1 = E(g(v).head)
//      g(v).remove(e1.i)
//      g(E(e1.i).oppo(v)).remove(e1.i)
//      val e2 = E(g(v).head)
//      g(v).remove(e2.i)
//      g(E(e2.i).oppo(v)).remove(e2.i)
//
//      debug(s"passOver($v) e1:$e1 e2:$e2")
//
//      deg(v) -= 2
//      deg(e1.oppo(v)) -= 1
//      deg(e2.oppo(v)) -= 1
//
//      addEntry(v)
//      addEntry(e1.oppo(v))
//      addEntry(e2.oppo(v))
//
//      ans += e1.oppo(v) -> v
//      ans += v -> e2.oppo(v)
//    }

    while(!q.isEmpty) {
      val e = q.poll()
      if (deg(e.v) == e.deg) {
        e.deg match {
          case 1 =>
            val edge = E(g(e.v).head)
            val u = edge.oppo(e.v)
            outward2(u, edge)

          case _ =>
            outward2(e.v, E(g(e.v).head))

//          case 3 =>
//            passOver(e.v)
        }
      }
    }

    assert(ans.length == M)
    REP(M) { i =>
      val e = ans(i)
      out.println(s"${e._1+1} ${e._2+1}")
    }

    true
  }

  def solve(): Unit = {
    if (!solve2()) out.println(-1)
  }
}