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;
class Program
{
    static void Main(string[] args)
    {
            // 入力
            string[] sS = System.Console.ReadLine().Trim().Split(' ');
            long iN = long.Parse(sS[0]);
            long iM = long.Parse(sS[1]);
            List<ClsShp> lstShp = new List<ClsShp>();
            for (int i = 0; i < iN; i++)
            {
                sS = System.Console.ReadLine().Trim().Split(' ');
                ClsShp oneUsr = new ClsShp();
                oneUsr.iA = long.Parse(sS[0]);
                oneUsr.iB = long.Parse(sS[1]);
                lstShp.Add(oneUsr);
            }

            System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
            sw.Start();

            // 解析
            lstShp.Sort(delegate (ClsShp mca1, ClsShp mca2) { return (int)(mca1.iA - mca2.iA); });        // 値段の昇順
            long iOut = 0;
            long iCnt = 0;
            for (int i = 0; i < lstShp.Count; i++)
            {
                if (iCnt + lstShp[i].iB < iM) { iOut += lstShp[i].iA * lstShp[i].iB; iCnt += lstShp[i].iB; }
                else { iOut += lstShp[i].iA * (iM - iCnt); iCnt = iM; break; }
            }

            // 出力
            System.Console.WriteLine("{0}", iOut.ToString());

            sw.Stop();
#if DEBUG
            Console.WriteLine();
            Console.WriteLine("{0}", sw.Elapsed.ToString());
            Console.WriteLine("続行するには何かキーを押してください．．．");
            Console.ReadKey();
#endif
    }

        // 店情報
        private class ClsShp
        {
            public long iA { get; set; } = 0;       // 値段
            public long iB { get; set; } = 0;       // 制限
        }

}