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 ConsoleApp1
{
    class Program
    {
        struct Store
        {
            public long Price { get; }
            public int Count { get; }

            public Store(long price, int count)
            {
                Price = price;
                Count = count;
            }
        }

        static void ReadInt2(out int a, out int b)
        {
            var array = Console.ReadLine().Split().Select(s => int.Parse(s)).ToArray();
            a = array[0];
            b = array[1];
        }

        static void ReadLong2(out long a, out long b)
        {
            var array = Console.ReadLine().Split().Select(s => long.Parse(s)).ToArray();
            a = array[0];
            b = array[1];
        }

        static Store ReadStore()
        {
            long price, count;
            ReadLong2(out price, out count);
            return new Store(price, (int)count);
        }

        static void Main(string[] args)
        {
            int n, m;
            ReadInt2(out n, out m);

            var stores = (from _ in Enumerable.Repeat(0, n)
                          let store = ReadStore()
                          orderby store.Price
                          select store).ToArray();

            long answer = 0;
            int remain = m;
            foreach (var store in stores)
            {
                if (remain <= 0)
                {
                    break;
                }

                int count = Math.Min(remain, store.Count);
                answer += store.Price * count;
                remain -= count;
            }

            Console.WriteLine(answer);
        }
    }
}