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;

namespace kyoupuro
{
    class MainClass
    {
        public static void Main()
        {
            MainClass mainClass = new MainClass();
            mainClass.Start();
        }

        public void Start()
        {
            var line = Console.ReadLine().Split(' ');
            Int64 N = Int64.Parse(line[0]), M = Int64.Parse(line[1]);
            var input = new List<List<Int64>>();
            Int64 count = 0;
            Int64 yen = 0;

            for (int i = 0; i < N; i++)
            {
                var tmp = Console.ReadLine().Split(' ').Select(Int64.Parse);
                input.Add(new List<Int64>(tmp));
            }

            var after = input.OrderBy(x => x[0]).ToArray();

            for (int i = 0; count < M;i++){
                //Console.WriteLine(after[i][0]);

                if(count + after[i][1] <= M) {
                    count += after[i][1];
                    yen += after[i][0] * after[i][1];
                } else {
                    var tmp = M - count;
                    count += tmp;
                    yen += tmp * after[i][0];
                }

                //Console.WriteLine(count);
            }

            Console.WriteLine(yen);
        }
    }
}