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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
fscanf(STDIN, "%d %d", $n, $max);
for( $i=0; $i<$n; $i++ )
{
  list( $price, $num )  = explode(" ", trim(fgets(STDIN)) );
  $shopList[] = array( "price"=>$price, "num"=>$num );
  $priceList[] = $price;
}
array_multisort( $shopList, SORT_ASC, $priceList );

$sum = 0;
foreach( $shopList as $shop )
{
  if( $max < $shop["num"] )
  {
    $sum += $max*$shop["price"];
    $max = 0;
  }
  else
  {
    $sum += $shop["num"]*$shop["price"];
    $max -= $shop["num"];
  }

  if( $max == 0 ) { break; }
}

echo $sum;
?>