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;

static class CodeJam
{
    static long Solve(int N, int M, Tuple<long, int>[] shops)
    {
        long ans = 0;
        Array.Sort(shops);
        foreach (var shop in shops)
        {
            int m = Math.Min(shop.Item2, M);
            ans += shop.Item1 * m;
            M -= m;
            if (M == 0) { break; }
        }
        return ans;
    }

    static void Main(string[] args)
    {
        int[] V1 = Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
        int N = V1[0], M = V1[1];
        var shops = new Tuple<long, int>[N];
        for (int i = 0; i < N; ++i)
        {
            long[] V2 = Array.ConvertAll(Console.ReadLine().Split(), long.Parse);
            shops[i] = new Tuple<long, int>(V2[0], (int)V2[1]);
        }
        Console.WriteLine(Solve(N, M, shops));
    }
}