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

            //   Tuple<int, int>[] tupleArray = new Tuple<int, int>[N];
            List<Tuple<long, long>> list = new List<Tuple<long, long>>();

            for (var i = 0; i < N; i++)
            {
                input = Console.ReadLine().Split(' ');
                list.Add(new Tuple<long, long>(long.Parse(input[0]), long.Parse(input[1])));
            }

            list = list.OrderBy(i => i.Item1).ToList();

            long sum = 0;
            long count = M;
            foreach(var element in list)
            {
                if (count == 0) break;

                var currentCount = Math.Min(count, element.Item2);
                sum += element.Item1 * currentCount;
                count -= currentCount;
            }

            Console.WriteLine(sum);
        }
    }
}