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.Collections.Generic;
class P{
    class S{
        public long A=0,B=0;
        public S(long[] a){
            A=a[0];
            B=a[1];
        }
    }
    static void Main(){
        var l=Console.ReadLine().Split().Select(long.Parse).ToArray();
        long ans=0,count=0;
        var g=new S[l[0]];
        for(int i=0;i<l[0];i++)g[i]=new S(Console.ReadLine().Split().Select(long.Parse).ToArray());
        g=g.OrderBy(x=>x.A).ToArray();
        for(int i=0;count<l[1];count++){
            ans+=g[i].A;
            g[i].B--;
            if(g[i].B==0)i++;
        }
        Console.WriteLine(ans);
    }
}