import java.io.BufferedReader
import java.io.InputStreamReader
import scala.collection.mutable

object test {
  def main(args: Array[String]) {
    val inputStreamReader = new InputStreamReader(System.in)
    val in1 = new BufferedReader(inputStreamReader)
    var n: Int = 0
    var k: Int = 0
    var str = in1.readLine().split(" ")
    n = str(0).toInt
    k = str(1).toInt

    var mas = new Array[(Long, Long)](n)
    for (i <- 0 until n) {
      str = in1.readLine().split(" ")
      mas(i) = (str(1).toInt, str(0).toInt)
    }

    def sorted(x: (Long, Long), y: (Long, Long)): Boolean = {
      x._1 > y._1
    }

    mas = mas.sortWith((x, y) => sorted(x, y))

    var heap = new Array[Long](n);
    var size: Int = 0

    var lengthOfKMax: Long = 0
    var answer: Long = 0

    def Push(value: Long): Boolean = {
      var index: Int = size
      size = size + 1
      heap(index) = value
      var par: Int = 0
      var tmp: Long = 0
      while (index > 0) {
        par = (index - 1) >> 1
        if (heap(index) > heap(par)) {
          tmp = heap(index)
          heap(index) = heap(par)
          heap(par) = tmp
          index = par
        } else {
          index = 0
        }
      }
      return true
    }

    def PollFirst(): Long = {
      var answer: Long = heap(0)
      var index: Int = 0
      var flag: Boolean = true
      size = size - 1
      heap(0) = heap(size)
      var nxt: Int = 0
      var tmp: Long = 0
      var left: Int = 0
      var right: Int = 0
      while (flag) {
        nxt = index
        left = (index << 1) + 1
        right = (index << 1) + 2
        if (left < size) {
          if (heap(left) > heap(index)) {
            nxt = left
          }
        }
        if (right < size) {
          if (heap(right) > heap(nxt)) {
            nxt = right
          }
        }
        if (nxt != index) {
          tmp = heap(nxt)
          heap(nxt) = heap(index)
          heap(index) = tmp
          index = nxt
        } else {
          flag = false
        }
      }
      return answer
    }

    for (i <- 0 until n) {
      if(i + 1 > k){
        lengthOfKMax += mas(i)._2
        Push(-mas(i)._2)
        lengthOfKMax += PollFirst()
      } else {
        lengthOfKMax += mas(i)._2
        Push(-mas(i)._2)
      }
      if (lengthOfKMax * mas(i)._1 > answer) {
        answer = lengthOfKMax * mas(i)._1
      }
    }

    println(answer)

    in1.close()
  }
}