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;
using System.Collections.Generic;

namespace CompetitiveProgramming
{
    class Program
    {
        static void Main(string[] args)
        {
            var  AB_Array = new List<int[]>();
            string[] n_m = Console.ReadLine().Split(' ');
            int n = int.Parse(n_m[0]);
            int m = int.Parse(n_m[1]);

            for (var i = 0; i < n; i++)
            {
                string[] A_B = Console.ReadLine().Split(' ');

                int a = int.Parse(A_B[0]);
                int b = int.Parse(A_B[1]);
                int[] ab = { a, b };
                AB_Array.Add(ab);
            }
            for (var start = 1; start < n; start++)
            {
                for (var end = n - 1; end >= start; end--)
                {
                    if(AB_Array[end-1][0] > AB_Array[end][0])
                    {
                        int[] tmp = AB_Array[end - 1];
                        AB_Array[end - 1] = AB_Array[end];
                        AB_Array[end] = tmp;
                    }
                }
            }

            int totalPrice = 0;
            for (var i = 0; i < n;i++)
            {
                if(m - AB_Array[i][1] >= 0)
                {
                    m -= AB_Array[i][1];
                    totalPrice += AB_Array[i][0] * AB_Array[i][1];
                }else{
                    totalPrice += AB_Array[i][0] * m;
                    break;
                }
            }
            Console.WriteLine(totalPrice);
        }


    }
}