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;
public class State
{

    public struct AB
    {
        public int A;
        public int B;

        public AB(int a,int b)
        {
            this.A = a;
            this.B = b;

        }
    }

    public static void Main()
    {
        int N, M;
        var split = Console.ReadLine().Split().Select(int.Parse).ToArray();
        N = split[0];
        M = split[1];
        AB[] ab = new AB[N];
        for (long i = 0; i < N; i++)
        {
            split = Console.ReadLine().Split().Select(int.Parse).ToArray();

            ab[i] = new AB(split[0], split[1]);
        }
        Array.Sort(ab,comparer);
        long store = 0;
        int need = M;
        int[] buy = new int[N];

        while (need >0)
        {
            if (ab[store].B <= need)
            {
                buy[store] = ab[store].B;
                need -= ab[store].B;
                store++;
            }
            else
            {
                buy[store] = need;
                break;
            }
        }
        long pay = 0;
        for (int i = 0; i < N; i++)
        {
            if (buy[i] == 0) break;
            long temp = ab[i].A;
            temp *= buy[i];
            pay += temp;
        }
        Console.WriteLine(pay);

    }

    public static int comparer(AB a,AB b)
    {
        return a.A.CompareTo(b.A);
    }



}