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: Wrong Answer)

"use strict"
function Main(stdin){
stdin = stdin.trim().split("\n").reverse();
let stdout = [];
function input(){return stdin.pop()};
function print(x){stdout.push(x)};
function mapint(x){return x.map(n=>parseInt(n))};
function mapfloat(x){return x.map(n=>parseFloat(n))};
function max(x){return x.reduce(function(a,b){ return Math.max(a,b) }) }
function min(x){return x.reduce(function(a,b){ return Math.min(a,b) }) }
function sum(x){return x.reduce(function(a,b){ return a+b }) }
function sorting(x, index){x.sort((a, b) => a[index] - b[index])}
function all(x){return x.reduce(function(a,b){ return a && b }) }
function any(x){return x.reduce(function(a,b){ return a || b }) }
function inlist(x, xs){return xs.includes(x)}
function inset(x, xs){return xs.has(x)}
function list(iterable){return Array.from(iterable)}
function arrayInt(iterable){return Array.from(iterable, x => parseInt(x))}
function tile(val, len){return Array(len).fill(val)}
for (let i = 0; i < 10; i++){ break; }
const range = (start, stop, step) => Array.from(
    { length: (stop - start) / step }, (_, i) => start + (i * step));
/*START*/


let nm = mapint(input().split(" "))
let n = nm[0]
let m = nm[1]
let x = [];
for (let i = 0; i < n; i++){
    x.push(mapint(input().split(" ")))
}
sorting(x, 0)


let cost = 0
for (let i = 0; i < 10; i++){
    cost += x[i][0] * Math.min(m, x[i][1])
    m -= x[i][1]
    if (m < 1){ break;  }
}
print(cost)



/*END*/
stdout = stdout.join("\n");
console.log(stdout);
return stdout
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"))