Problem:
Score : 300 points

Problem Statement

Hearing that energy drinks increase rating in those sites, Takahashi decides to buy up M cans of energy drinks.

There are N stores that sell energy drinks. In the i-th store, he can buy at most B_i cans of energy drinks for A_i yen (the currency of Japan) each.

What is the minimum amount of money with which he can buy M cans of energy drinks?

It is guaranteed that, in the given inputs, a sufficient amount of money can always buy M cans of energy drinks.

Constraints

All values in input are integers.

1 \leq N, M \leq 10^5

1 \leq A_i \leq 10^9

1 \leq B_i \leq 10^5

B_1 + ... + B_N \geq M

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
\vdots
A_N B_N

Output

Print the minimum amount of money with which Takahashi can buy M cans of energy drinks.

Sample Input 1

2 5
4 9
2 4

Sample Output 1

12

With 12 yen, we can buy one drink at the first store and four drinks at the second store, for the total of five drinks. However, we cannot buy 5 drinks with 11 yen or less.

Sample Input 2

4 30
6 18
2 5
3 10
7 9

Sample Output 2

130

Sample Input 3

1 100000
1000000000 100000

Sample Output 3

100000000000000

The output may not fit into a 32-bit integer type.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;

namespace abc121_c
{
  class Program
  {
    static void Main(string[] args)
    {
      var n = CIn.ReadInt();
      var m = CIn.ReadInt();
      var l = new SortedDictionary<int, int>();
      var c = 0L;
      var d = 0;
      
      for (var i = 0; i < n; i++)
      {
        var key = CIn.ReadInt();

        if (!l.ContainsKey(key))
        {
          l[key] = 0;
        }

        l[key] += CIn.ReadInt();
      }

      foreach (var e in l)
      {
        var v = e.Value;
        while (v-- > 0)
        {
          c += e.Key;
          if (++d == m)
          {
            Console.WriteLine(c);
            return;
          }
        }
      }
    }
  }

  class CIn
  {
    private static readonly Queue<string> fieldBuffer = new Queue<string>();

    public static string Read()
    {
      if (fieldBuffer.Count == 0)
      {
        foreach (var field in Console.ReadLine().Split())
        {
          fieldBuffer.Enqueue(field);
        }
      }

      return fieldBuffer.Dequeue();
    }

    public static int ReadInt() => int.Parse(Read());
    public static long ReadLong() => long.Parse(Read());
    public static double ReadDouble() => double.Parse(Read());

    public static string[] ReadStringArray(long n)
    {
      var array = new string[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = Read();
      }

      return array;
    }

    public static int[] ReadIntArray(long n)
    {
      var array = new int[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = ReadInt();
      }

      return array;
    }

    public static long[] ReadLongArray(long n)
    {
      var array = new long[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = ReadLong();
      }

      return array;
    }
    public static double[] ReadDoubleArray(long n)
    {
      var array = new double[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = ReadDouble();
      }

      return array;
    }
  }
}