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;
using System.Threading.Tasks;

namespace AtCorder
{
    class Program
    {
        static void Main(string[] args)
        {
            var in1 = Console.ReadLine().Split(' ').Select(x => decimal.Parse(x.ToString())).ToList();
            var N = in1[0];
            var M = in1[1];

            var ABList = new List<AB>();
            for (int i = 0; i < N; i++)
            {
                var in2 = Console.ReadLine().Split(' ').Select(x => decimal.Parse(x.ToString())).ToList();
                ABList.Add(new AB(in2[0],in2[1]));
            }
            ABList = ABList.OrderBy(x => x.A).ToList();

            decimal ans = 0;
            decimal m = 0;
            for (int i = 0; i < N; i++)
            {
                if(ABList[i].B + m <= M)
                {
                    ans += ABList[i].A * ABList[i].B;
                    m += ABList[i].B;
                    continue;
                }

                ans += (M - m)* ABList[i].A;
                break;
            }
            Console.WriteLine(ans.ToString());
        }

        public class AB
        {
            public decimal A { get; set; }
            public decimal B { get; set; }

            public AB(decimal a , decimal b)
            {
                A = a;
                B = b;
            }
        }
    }
}