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",$N,$M);

$ab_li=array();
for($i=0;$i<$N;$i++){
 $ab_li[] = explode(" ",trim(fgets(STDIN)));
}

sort($ab_li);

//print_r($ab_li);
$sum_money=0;
$sum_juce=0;
foreach($ab_li as $ab){
  //全部買いたいとき
  if(($ab[1]+$sum_juce)<=$M){
    $sum_juce+=$ab[1];
    $sum_money+=$ab[0]*$ab[1];
  }else{
    $sum_money+=$ab[0]*($M-$sum_juce);
    $sum_juce+=($M-$sum_juce);
  } 
 
  if($sum_juce>=$M){
    echo $sum_money."\n";
    break;
  }
}