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: Wrong Answer)

using System.Collections.Generic;
using System.Linq;
using System;
using static System.Console;

class Program
{
  static void Main() => new Program().solve();

  void solve()
  {
    var param = ReadLine().Split(' ').Select(int.Parse).ToArray();
    var n = param[0];
    var m = param[1];

    var d = Enumerable.Range(0, n)
      .Select(_ => ReadLine().Split(' ').Select(int.Parse).ToArray())
      .OrderBy(e => e[0])
      .ToArray();

    var acc = 0L;
    var cnt = 0L;
    foreach(var e in d)
    {
      if(cnt + e[1] >= m)
      {
        acc += e[0] * (m - cnt);
        break;
      }

      cnt += e[1];
      acc += e[0] * e[1];
    }

    var ans = acc;
    WriteLine(ans);
  }
}