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.Linq;
using System.Collections.Generic;
using System.Collections;
public class Program {
  public static void Main() {
    int[] line1 = ReadIntArray();
    int N = line1[0], M = line1[1];
    Dictionary<int,int> dic = new Dictionary<int,int>();
    // key is price, value is quantity.
    for (int i = 0; i < N; i++) {
      int[] line2 = ReadIntArray();
      if (dic.ContainsKey(line2[0])) {
        dic[line2[0]] += line2[1];
      } else {
        dic.Add(line2[0], line2[1]);
      }
    }
    var _dic = dic.OrderBy(x => x.Key);
    long res = 0;
    int c = 0;
    foreach (var i in _dic) {
      if (c + i.Value > M) {
        res += (M-c) * (long)i.Key;
        break;
      } else {
        res += i.Key * (long)i.Value;
        c += i.Value;
      }
    }
    Console.WriteLine(res);
  }

  private static string ReadStr() { return Console.ReadLine(); }
  private static int ReadInt() { return int.Parse(Console.ReadLine()); }
  private static double ReadDouble() { return double.Parse(Console.ReadLine()); }
  private static long ReadLong() { return long.Parse(Console.ReadLine()); }
  private static char ReadChar() { return Console.ReadLine()[0]; }
  private static string[] ReadStringArray() { return Console.ReadLine().Split(' '); }
  private static int[] ReadIntArray() { return Console.ReadLine().Split(' ').Select(int.Parse).ToArray(); }
  private static double[] ReadDoubleArray() { return Console.ReadLine().Split(' ').Select(double.Parse).ToArray(); }
  private static long[] ReadLongArray() { return Console.ReadLine().Split(' ').Select(long.Parse).ToArray(); }
  private static char[] ReadCharArray() { return Console.ReadLine().ToArray(); }
  private static string[] ReadCharStringArray() { return Console.ReadLine().ToCharArray().Select(c => c.ToString()).ToArray(); }
}