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;
using System.Text;

class Program
{
    static void Main(string[] args)
    {

        // 入力
        long[] nm = Console.ReadLine().Split().Select(long.Parse).ToArray();
        var n = nm[0];
        var m = nm[1];

        long[] a = new long[n];
        long[] b = new long[n];
        for (int i = 0; i < n; i++)
        {
            long[] tmp = Console.ReadLine().Split().Select(long.Parse).ToArray();
            a[i] = tmp[0];
            b[i] = tmp[1];
        }

        Array.Sort(a, b);
        long count = 0;
        long ans = 0;
        for (int i = 0; i < n; i++)
        {
            count += b[i];
            if (count < m)
            {
                ans += a[i] * b[i];
            }
            else if (count == m)
            {
                ans += a[i] * b[i];
                break;
            }
            else
            {
                ans += a[i] * (m - (count - b[i]));
                break;
            }
        }

        Console.WriteLine(ans);
    }
}