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 static System.Console;

namespace AtCoder
{
    class Program
    {
        static void Main()
        {
            var raw = ReadLine().Split().Select(int.Parse).ToArray();
            var N = raw[0];
            var M = raw[1];

            var drink = new Drink[N];
            for (int i = 0; i < N; i++)
            {
                var raw2 = ReadLine().Split().Select(long.Parse).ToArray();
                drink[i].Price = raw2[0];
                drink[i].Number = raw2[1];
            }

            drink = drink.OrderBy(d => d.Price).ToArray();

            var ans = 0L;
            var sum = 0L;
            foreach (var item in drink)
            {
                if (sum + item.Number >= M)
                {
                    WriteLine(ans + (M - sum) * item.Price);
                    return;
                }
                else
                {
                    ans += item.Number * item.Price;
                    sum += item.Number;
                }
            }
        }
    }

    struct Drink
    {
        public long Number;
        public long Price;
    }
}