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;
using System.Collections.Generic;
using System.IO;
using System.Linq;

namespace C
{
    public class Program
    {
        static void Main(string[] args)
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
            Solve();
            Console.Out.Flush();
        }

        public static void Solve()
        {
            var nm = Console.ReadLine().Trim().Split(' ').Select(long.Parse).ToArray();
            var shops = new Pair[nm[0]];
            for (var i = 0; i < nm[0]; i++)
            {
                var ab = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
                shops[i] = new Pair { Price = ab[0], Count = ab[1] };
            }
            shops = shops.OrderBy(x => x.Price).ToArray();
            var money = 0L;
            var current = 0L;
            for (var i = 0; i < shops.Length; i++)
            {
                if (current < nm[1])
                {
                    current += shops[i].Count;
                    money += shops[i].Price * shops[i].Count;
                }
                if (current >= nm[1])
                {
                    money -= shops[i].Price * (current - nm[1]);
                    break;
                }
            }
            Console.WriteLine(money);
        }

        public struct Pair
        {
            public long Price { get; set; }
            public long Count { get; set; }
        }
    }
}