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.ComponentModel;
using System.Linq;
using System.Text;
using static System.Console;

namespace ABC121C
{
    static class Program
    {
        static void Main(string[] args)
        {
            var line = ReadLine().SplitTryParseToList<long>();
            var a = new List<List<long>>();
            for (long i = 0; i < line[0]; i++)
            {
                a.Add(ReadLine().SplitTryParseToList<long>());

            }

            var aa = a.OrderBy(x => x[0]).ToList();
            long katta = 0;
            long kane = 0;
            int index = 0;
            while (katta < line[1])
            {
                var kau = (katta + aa[index][1] > line[1] ? line[1] - katta : aa[index][1]);
                kane += (kau * aa[index][0]);
                katta += kau;
                index++;
            }

            WriteLine(kane);

        }

        public static T TryParse<T>(this String input)
        {
            try
            {
                var converter = TypeDescriptor.GetConverter(typeof(T));
                if (converter != null)
                {
                    return (T)converter.ConvertFromString(input);
                }
                else
                {
                    throw new InvalidCastException("");
                }
            }
            catch
            {
                throw new InvalidCastException(typeof(T) + " is not supported.");
            }
        }

        public static List<T> SplitTryParseToList<T>(this String input)
        {
            return input.Split().Select(n => n.TryParse<T>()).ToList();
        }

        public static List<T> ListSwap<T>(this List<T> list, Int32 index1, Int32 index2)
        {
            var t = list[index1];
            list[index1] = list[index2];
            list[index2] = t;
            return list;
        }

        public static List<Tuple<T, int>> DuplicateCount<T>(this IEnumerable<T> list)
        {
            return list
                .GroupBy(i => i)
                .Where(g => g.Any())
                .Select(g => Tuple.Create(g.Key, g.Count()))
                .ToList();
        }
        public static List<Tuple<T, int>> DuplicateSort<T>(this IEnumerable<Tuple<T, int>> list)
        {
            return list.OrderByDescending((x) => x.Item2).ToList();
        }
        public static List<T> ReadLineOne<T>(int n)
        {
            var list = new List<T>();
            foreach (var i in Enumerable.Range(1, n))
            {
                list.Add(Console.ReadLine().TryParse<T>());
            }
            return list;
        }
        public static void PrintAll<T>(this IEnumerable<T> list)
        {
            foreach (var i in list)
            {
                Console.Write($"{i} ");
            }

            Console.WriteLine();
        }
    }
}