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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Threading;

class Progaram
{
    static void Main(String[] args)
    {
        var l = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
        var n = l[0];
        var m = l[1];

        var stores = new List<Store>();

        for (var i = 0; n > i; i++)
        {
            var li = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
            stores.Add(new Store(li[0], li[1]));
        }

        stores = stores.OrderBy(x => x.Price).ToList();

        var now = 0L;
        var res = 0L;
        while (now < m)
        {
            long f;
            if (now + stores[0].Ko > m)
            {
                f = m - now;
            }
            else
            {
                f = stores[0].Ko;
            }
            now += f;
            res += f * stores[0].Price;
            stores.RemoveAt(0);
        }
        Console.WriteLine(res);
    }
}

class Store
{
    public long Price { get; set; }
    public long Ko { get; set; }

    public Store(long price, long ko)
    {
        this.Price = price;
        this.Ko = ko;
    }
}