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;

namespace ABC121_C
{
    class Program
    {
        static void Main(string[] args)
        {
            var NM = Console.ReadLine().Split().Select(int.Parse).ToArray();
            var N = NM[0];
            var M = NM[1];

            var AB = new List<Model>();

            for (int i = 0; i < N; i++)
            {
                var ab = Console.ReadLine().Split().Select(long.Parse).ToArray();
                AB.Add(new Model() { A = ab[0], B = ab[1] });
            }

            AB = AB.OrderBy(x => x.A).ToList();

            long ans = 0;
            long sum = 0;

            var n = 0;
            while (sum < M)
            {
                if (ans + AB[n].A * AB[n].B < M)
                {
                    ans += AB[n].A * AB[n].B;
                    sum += AB[n].B;
                    AB[n].B = 0;
                }
                else
                {
                    ans += AB[n].A;
                    AB[n].B--;
                    sum++;
                }
                if (AB[n].B == 0) n++;
            }

            Console.WriteLine(ans);
        }
    }

    class Model
    {
        public long A { get; set; }
        public long B { get; set; }
    }
}