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;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("2 5");
            WillReturn.Add("4 9");
            WillReturn.Add("2 4");
            //12
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("4 30");
            WillReturn.Add("6 18");
            WillReturn.Add("2 5");
            WillReturn.Add("3 10");
            WillReturn.Add("7 9");
            //130
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("1 100000");
            WillReturn.Add("1000000000 100000");
            //100000000000000
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();

        int[] wkArr = { };
        Action<string> SplitAct = pStr =>
            wkArr = pStr.Split(' ').Select(X => int.Parse(X)).ToArray();

        SplitAct(InputList[0]);
        int M = wkArr[1];

        var ABList = new List<int[]>();
        foreach (string EachStr in InputList.Skip(1)) {
            SplitAct(EachStr);
            ABList.Add(wkArr);
        }

        long Answer = 0;
        long BoughtCnt = 0;
        foreach (int[] EachAB in ABList.OrderBy(X => X[0])) {
            for (int I = 1; I <= EachAB[1]; I++) {
                Answer += EachAB[0];
                if (++BoughtCnt == M) {
                    Console.WriteLine(Answer);
                    return;
                }
            }
        }
    }
}