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;

namespace AtCoderBegginerContest121
{
    class Program
    {
        static void Main(string[] args)
        {
            var inptNM = Console.ReadLine().Split(' ');
            int N = int.Parse(inptNM[0]);
            int M = int.Parse(inptNM[1]);

            List<EneDri> EneDriList= new List<EneDri> { };

            for(int i = 0; i < N; i++)
            {
                var inpt = Console.ReadLine().Split(' ');
                EneDriList.Add(new EneDri(Int64.Parse(inpt[0]), Int64.Parse(inpt[1])));
            }
            EneDriList.Sort((a,b) => (int)(a.Price - b.Price));

            long stockSum = 0;
            long priceSum = 0;

            foreach(EneDri enedri in EneDriList)
            {
                if (stockSum + enedri.Stock < M)
                {
                    priceSum = priceSum + enedri.Price * enedri.Stock;
                    stockSum = stockSum + enedri.Stock;
                }
                else{
                    priceSum = priceSum + enedri.Price * (M - stockSum);
                    break;
                }
            }
            Console.Write(priceSum);
        }
    }
    class EneDri { 
        public long Price { get; set; }
        public long Stock { get; set; }
        public EneDri(long price,long stock)
        {
            this.Price = price;
            this.Stock = stock;
        }
    }
}