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;
using System.Text;

namespace atcorder_BeginnerContest_121
{
    class Program
    {

        struct ShopData
        {
            public long price;
            public int cnt;
        }
        static void Main(string[] args)
        {
            int[] dataArr = Array.ConvertAll(Console.ReadLine().Split(' '), int.Parse);
            int shopCnt = dataArr[0];
            int getCnt = dataArr[1];

            List<ShopData> dataList = new List<ShopData>();

            for (int i = 0; i < shopCnt; i++)
            {
                long[] tmpArr = Array.ConvertAll(Console.ReadLine().Split(' '), long.Parse);
                ShopData sd = new ShopData();
                sd.price = tmpArr[0];
                sd.cnt = (int)tmpArr[1];
                dataList.Add(sd);
            }
            dataList.Sort((a, b) => a.price.CompareTo(b.price));

            long retPrice = 0;
            for (int i = 0; i < shopCnt; i++)
            {
                if (dataList[i].cnt < getCnt)
                {
                    retPrice += dataList[i].cnt * dataList[i].price;
                    getCnt -= dataList[i].cnt;
                }
                else
                {
                    retPrice += getCnt * dataList[i].price;
                    break;
                }
            }

            Console.WriteLine(retPrice);

        }
    }
}