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 JavaScript (node.js v5.12) (judge verdict: Accepted)

var lines = [];


var readline = require("readline");
var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.on('line', function (x) {
  lines.push(x);
});

rl.on('close', function () {
  var N = Number(lines[0].split(" ")[0]);
  var M = Number(lines[0].split(" ")[1]);

  lines.shift();

  var AB = lines.map(i => i.split(" ").map(i => Number(i)));
  AB.sort((a, b) => a[0] - b[0]);


  var ans = 0;
  for (var i = 0; i < N; i++) {
    var A = AB[i][0]; var B = AB[i][1];

    if (B < M) { ans += A * B; M -= B }
    else {
      ans += A * M; break;
    }
  }
  console.log(ans);
});