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 PHP (5.6.30) (judge verdict: Wrong Answer)

<?php
    $NM = explode(" ",trim(fgets(STDIN)));
    $N = $NM[0];
    $M = $NM[1];
    
    for ($i = 0; $i < $N; $i++) {
        $AB = explode(" ",trim(fgets(STDIN)));
        $price[] = $AB[0];
        $stock[$price[$i]] = $AB[1];
    }
    
    
    ksort($stock);
    $buy = 0;
    $ans = 0;
    foreach ($stock as $key => $val)
    {
        //この店で買う数
        $buy = min($M, $val);
        
        //金額をたす
        $ans += $buy * $key;
        
        //必要な個数を減らす
        $M -= $buy;
    }
    
    echo $ans;
    
?>