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;

namespace ABC121
{
    class C
    {
        static void Main(string[] args)
        {
            int[] h = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            int N = h[0], M = h[1];

            List<Drink> list = new List<Drink>();

            for (int i = 0; i < N; i++)
            {
                int[] g = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
                int A = g[0];
                int B = g[1];
                Drink d = new Drink(); d.price = A; d.num = B;
                list.Add(d);
            }

            list = list.OrderBy(x => x.price).ToList();

            long sum = 0;
            int count = 0;
            foreach (var v in list)
            {
                if (count + v.num > M)
                {
                    sum += (long)(M - count) * v.price;
                    count = M;
                }
                else
                {
                    sum += (long)v.num * v.price;
                    count += v.num;
                }

                if (count == M) break;
            }

            Console.WriteLine(sum);
        }

    }
    struct Drink
    {
        public int price;
        public int num;
    }
}