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;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var input = Console.ReadLine().Split(' ').Select(x => ulong.Parse(x)).ToArray();
            var n = (int)input[0];
            var m = input[1];
            Hoge[] hoges = new Hoge[n];
            for (int i = 0; i < n; i++)
            {
                input = Console.ReadLine().Split(' ').Select(x => ulong.Parse(x)).ToArray();
                hoges[i] = new Hoge {Foo = input[0], Bar = input[1]};
            }

            hoges = hoges.OrderBy(x => x.Foo).ToArray();
            ulong sum = 0;
            ulong count = 0;
            for (int i = 0; i < n; i++)
            {
                if (hoges[i].Bar + count > m)
                {
                    sum += (m - count) * hoges[i].Foo;
                    break;
                }

                sum += hoges[i].Foo * hoges[i].Bar;
                count += hoges[i].Bar;
            }

            Console.WriteLine(sum);
        }

        public class Hoge
        {
            public ulong Foo { get; set; }
            public ulong Bar { get; set; }
        }
    }
}