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: Time Limit Exceeded)

function main(input) {
    var i, j, A, ans=0
    var arg = input.split("\n")[0].split(" ")
    //A=new Array(arg[0])
    A=[]
    for (i=0;i<arg[0];++i){
        A.push(input.split("\n")[i+1].split(" "))
    }
    A.sort(function(a,b){return(a[0] - b[0])})
    i=0
    j=0
    while(j<Number(arg[1])){
        if (arg[1]-j < A[i][1]){
            ans += Number(A[i][0])*(arg[1]-j)
            j = arg[1]
        }else{
            ans += Number(A[i][0])*Number(A[i][1])
            j += Number(A[i][1])
            A[i][1]=0
            i++
        }
    }
    console.log(ans)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));