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;

class _3
{
    static void Main(string[] args)
    {
        string[] input = Console.ReadLine().Split(' ');
        int N = int.Parse(input[0]);
        int M = int.Parse(input[1]);

        var store = new Store[N];

        for(int i = 0; i < N; i++)
        {
            input = Console.ReadLine().Split(' ');
            store[i] = new Store();
            store[i].A = int.Parse(input[0]);
            store[i].B = int.Parse(input[1]);
        }

        Array.Sort(store);

        long money = 0;
        long count = 0;
        for(int i = 0; i < N;i++)
        {

            long n = 0;
            if(M < count + store[i].B)
            {
                n = M - count;
            } else
            {
                n = store[i].B;
            }
            money += n * store[i].A;
            count += n;
            
            if(count == M)
            {
                break;
            }
        }

        Console.WriteLine(money);
    }

    class Store : IComparable<Store>
    {
        public int A { get; set; }
        public int B { get; set; }

        public int CompareTo(Store other)
        {
            return A - other.A;
        }
    }
}