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;

class Program
{
    static void Main(string[] args)
    {
        var input = Console.ReadLine().Split(' ');
        var N = int.Parse(input[0]);
        var M = int.Parse(input[1]);
        var list = new List<Hoge>();
        
        for (var i = 0; i < N; i++) {
            var input2 = Console.ReadLine().Split(' ');
            list.Add(new Hoge());
            list[i].key = decimal.Parse(input2[0]);
            list[i].value = decimal.Parse(input2[1]);
        }
        
        var newList = list.OrderBy(x => x.key);
        decimal yen = 0;
        decimal cnt = 0;
        foreach (var item in newList) {
            if (cnt > 0) {
                if ((M - cnt) < item.value) {
                    yen += (M - cnt) * item.key;
                    break;
                } else {
                    yen += item.key * item.value;
                    cnt += item.value;
                }
            } else {
                if (item.value < M) {
                    yen += item.key * item.value;
                    cnt += item.value;
                } else {
                    yen = M * item.key;
                    break;
                }   
            }
        }
        
        Console.WriteLine(yen);
    }
}

public class Hoge {
    public decimal key { get; set; } 
    public decimal value { get; set; }
}