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.Linq;

class Program
{
    static void Main(string[] args)
    {
        int[] nm = Console.ReadLine()
            .Split()
            .Select(int.Parse)
            .ToArray();

        int n = nm[0];
        int m = nm[1];

        Shop[] shops = new Shop[n];
        for (int i = 0; i < n; i++)
        {
            int[] ab = Console.ReadLine()
                .Split()
                .Select(int.Parse)
                .ToArray();
            shops[i] = new Shop(ab[0], ab[1]);
        }
        Array.Sort(shops, (v1, v2) => (int)v1.Price - (int)v2.Price);

        long count = 0;
        long totalPrice = 0;
        int index = -1;
        for (int i = 0; i < n; i++)
        {
            if (count + shops[i].Quantity >= m)
            {
                index = i;
                break;
            }
            count += shops[i].Quantity;
            totalPrice += shops[i].Price * shops[i].Quantity;
        }

        totalPrice += shops[index].Price * (m - count);
        Console.WriteLine(totalPrice);
    }

    class Shop
    {
        public long Price;
        public long Quantity;
        public Shop(int price, int quantity)
        {
            this.Price = price;
            this.Quantity = quantity;
        }
    }
}