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;
using System.Collections.Generic;
using System.Linq;

namespace AtCoder
{
    public class MainClass
    {

        public static void Main(string[] args)
        {
            checked
            {
                var paramsList = Console.ReadLine().Split(' ').ToList();
                var n = int.Parse(paramsList[0]);
                var m = int.Parse(paramsList[1]);

                var list = EnumerablePlus
                    .Repeat(n, () => Console.ReadLine())
                    .Select(line => line.Split(' ').Select(s => int.Parse(s)).ToList())
                    .Select(l => new { bid = l[0], max = l[1] })
                    .ToList();

                list.Sort((a, b) => a.bid - b.bid);

                long res = 0;
                foreach (var shop in list)
                {
                    if (m <= shop.max)
                    {
                        res += ((long)shop.bid * m);
                        break;
                    }

                    res += (long)shop.bid * shop.max;
                    m -= shop.max;
                }

                Console.WriteLine(res);
            }
        }

    }

    public static class EnumerablePlus
    {
        ///<summary>出力文字列に変換。</summary>
        public static string MkString<T>(this IEnumerable<T> list)
        {
            return list.Aggregate(
                new System.Text.StringBuilder(),
                (a, b) => ((a.Length > 0) ? a.Append(' ') : a).Append(b)
                ).ToString();
        }

        ///<summary>添字付きfor。</summary>
        public static void ForEach<T>(this IEnumerable<T> list, Action<T, int> a)
        {
            int i = 0;
            foreach (var item in list)
            {
                a(item, i);
            }
        }

        ///<summary>指定回数デリゲートを呼び出す。</summary>
        public static IEnumerable<T> Repeat<T>(int n, Func<T> f)
        {
            for (int i = 0; i < n; i++)
            {
                yield return f();
            }
        }
    }

}