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 = 1_000_000_007

fun findCycle(n: Int, m: Int, from: IntArray, to: IntArray): List<Int>? {
  data class Edge(val i: Int, val v: Int) // 外に出せ
  val que = IntArray(n + 10)
  val nodeFlag = IntArray(n)
  val visitedEdge = BooleanArray(m)
  val g = Array(n){ mutableListOf<Edge>()}
  for (i in 0 until m) {
    g[from[i]].add(Edge(i, to[i]))
    g[to[i]].add(Edge(i, from[i]))
  }
  fun dfs(rt: Int): List<Int>?  {
    fun step(pos: Int, v: Int): List<Int>? {
      que[pos] = v
      nodeFlag[v] = 1
      for (i in g[v].indices) {
        val e = g[v][i]
        if (visitedEdge[e.i]) continue
        visitedEdge[e.i] = true
        when(nodeFlag[e.v]) {
          1 -> {
            val ix = que.indexOf(e.v)
            val ans = mutableListOf<Int>()
            for (j in ix..pos) {
              ans += que[j]
            }
            return ans
          }
          0 -> {
            val res = step(pos + 1, e.v)
            if (res != null) return res
          }
          else -> {}
        }
      }
      nodeFlag[v] = 2
      return null
    }

    return step(0, rt)
  }

  for (u in 0 until n) {
    if (nodeFlag[u] == 0) {
      val res = dfs(u)
      if (res != null) return res
    }
  }
  return null
}

fun packUGraph(n: Int, from: IntArray, to: IntArray): Array<IntArray> {
  val p = IntArray(n)
  val m = from.size
  for (i in 0 until m) {
    ++p[from[i]]
    ++p[to[i]]
  }
  val g = Array(n){IntArray(p[it])}
  for (i in 0 until m) {
    g[from[i]][--p[from[i]]] = to[i]
    g[to[i]][--p[to[i]]] = from[i]
  }
  return g
}
data class Point(val h: Int, val w: Int)
class Solver(stream: InputStream, private val out: java.io.PrintWriter) {
  private val reader = BufferedReader(InputStreamReader(stream), 32768)

  fun solve() {
    val th = Thread(null, Runnable {
      solve2()
    }, "solve", 1 shl 26)
    th.start()
    th.join()
  }

  fun solve2() {
    val (H, W, M) = na(3)
    val (from, to) = na2(M, -1)
    for (i in 0 until M) {
      to[i] += H
    }
    val toP = Array(M){
      Pair(Point(from[it], to[it]), it)
    }.toMap()
    val cycle = findCycle(H + W, M, from, to)
    if (cycle == null) {
      out.println(-1)
      return
    }

    debug{"cycle:$cycle"}

    val N = cycle.size
    var i = if (cycle.first() < H) 0 else 1 // Hスタートかどうか
    val ans = IntArray(N)
    for (j in 0 until N) {
      val i2 = (i + 1) % N
      val p = if (j % 2 == 0) Point(cycle[i], cycle[i2]) else Point(cycle[i2], cycle[i])
      debug{"$j $p"}
      ans[j] = toP[p]!!
      i = i2
    }
    out.println(N)
    out.println(ans.map{it+1}.joinToString(" "))
  }













































  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<IntArray> {
    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<LongArray>) {
    if (isDebug) {
      for (a in A) {
        debug(a)
      }
    }
  }
  private inline fun debugDim(A: Array<IntArray>) {
    if (isDebug) {
      for (a in A) {
        debug(a)
      }
    }
  }
  private inline fun debugDim(A: Array<BooleanArray>) {
    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())}

  companion object {
    // TestRunnerから呼びたいので単純なmainじゃだめ
    fun main() {
      val out = java.io.PrintWriter(System.out)
      Solver(System.`in`, out).solve()
      out.flush()
    }
  }
}

/**
 * judgeから呼ばれる
 */
fun main() = Solver.main()