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: Wrong Answer)

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

namespace test01
{
    class Program
    {
        static void Main(string[] args)
        {
            var line = U.GetLineLongParse();
            var N = line[0]; // 店の数
            var M = line[1]; // 栄養ドリンクの本数

            List<MiseInfo> miseInfos = new List<MiseInfo>();
            for (int i = 0; i < N; i++)
            {
                var line2 = U.GetLineLongParse();
                miseInfos.Add(new MiseInfo() { price = line2[0], cnt = line2[1] });
            }

            long ans = 0;
            long c = M;
            miseInfos = miseInfos.OrderBy(e => e.price).ToList();
            foreach (var miseInfo in miseInfos)
            {
                var price = miseInfo.price;
                var cnt = miseInfo.cnt;
                if (cnt == c)
                {
                    ans = price * cnt;
                    break;
                }
                else if (miseInfo.cnt < c)
                {
                    ans = ans + price * cnt;
                    c = c - cnt;
                }
                else
                {
                    ans = ans + price * c;
                    c = c - cnt;
                }

                if (c <= 0) { break; }
            }

            Console.WriteLine(ans);
            
        }

        class MiseInfo
        {
            public long price { get; set; }
            public long cnt { get; set; }
        }

        class U
        {
            public static string GetLine() { return Console.ReadLine(); }

            public static List<int> GetLineIntParse() { return Console.ReadLine().Split().Select(e => int.Parse(e)).ToList(); }

            public static List<long> GetLineLongParse() { return Console.ReadLine().Split().Select(e => long.Parse(e)).ToList(); }
        }
    }
}