package chelper;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Random;

import io.InputReader;
import io.OutputWriter;
import misc.GCJSolution;
import misc.SimpleSavingChelperSolution;


public class TaskA extends GCJSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  class Pancake implements Comparable<Pancake> {
    final int r;
    final int h;

    public Pancake(int r, int h) {
      this.r = r;
      this.h = h;
    }

    @Override
    public int compareTo(Pancake o) {
      int t = -Integer.compare(r, o.r);
      if (t != 0) {
        return t;
      }
      return Integer.compare(h, o.h);
    }

    double sides() {
      return 2 * Math.PI * r * h;
    }

    double area() {
      return Math.PI * r * r;
    }

    double total() {
      return sides() + area();
    }

    @Override
    public String toString() {
      return "Pancake{" +
          "r=" + r +
          ", h=" + h +
          '}';
    }
  }

  @Override
  public void solve(int testNumber) {
    int n = in.nextInt();
    int k = in.nextInt();

    List<Pancake> pancakes = new ArrayList<>();

    Random random = new Random();

    for (int i = 0; i < n; i++) {
      pancakes.add(new Pancake(in.nextInt(), in.nextInt()));

//      pancakes.add(new Pancake(random.nextInt(100000), random.nextInt(100000)));
    }

    Collections.sort(pancakes);

    double[][] dp = new double[n + 1][n];

    for (int i = 0; i < n; i++) {
      for (int j = 0; j < n; j++) {
        dp[i][j] = -1;
      }
    }

//    for (Pancake pancake : pancakes) {
//      System.out.println(pancake);
//    }

    Arrays.fill(dp[0], 0);

    for (int addI = 0; addI < n; addI++) {
      Pancake add = pancakes.get(addI);

      double addTotal = add.total();
      double addArea = add.area();

      for (int i = n - 1; i >= 0; i--) {
        for (int j = 0; j < n; j++) {
          if (dp[i][j] < 0) {
            continue;
          }

          double newRes = dp[i][j] + addTotal;
          if (i != 0) {
            newRes -= addArea;
          }

          if (newRes > dp[i + 1][addI]) {
//            System.out.println("set " + (i + 1) + " " + j + " + " + addI + " = " + newRes);
            dp[i + 1][addI] = newRes;
          }
        }
      }
    }

    double max = -1;
    for (int i = 0; i < n; i++) {
      max = Math.max(max, dp[k][i]);
    }
//    System.out.println();

    out.println(max);
  }
}