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;
using System.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //[summary]C - Energy Drink Collector
            var line = ReadLine();
            int n = line[0];
            int m = line[1];

            var shops = new List<Shop>();

            for(int i = 0; i < n; i++)
            {
                shops.Add(new Shop());
            }

            //ドリンクの値段の安い順に店をソート
            shops = shops.OrderBy(x => x.Price).ToList();

            //買わなければいけないドリンクの本数
            long remainder = m;
            long money = 0;

            foreach (var s in shops)
            {
                if (remainder >= s.Stock)
                {
                    money += s.Price * s.Stock;
                    remainder -= +s.Stock;
                }
                else
                {
                    money += s.Price * remainder;
                    break;
                }
            }

            Console.WriteLine(money);
        }

        static List<int> ReadLine()
        {
            var line = Console.ReadLine().TrimEnd();
            var array = line.Split(' ');
            var numbers = new List<int>();

            foreach (var s in array)
            {
                int num = int.Parse(s);
                numbers.Add(num);
            }

            return numbers;
        }
    }

    class Shop
    {
        public long Price { set; get; }
        public long Stock { set; get; }

        public Shop()
        {
            var line = Console.ReadLine();
            var s = line.Split(' ');

            Price = long.Parse(s[0]);
            Stock = long.Parse(s[1]);
        }
    }
}