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 Go (1.6) (judge verdict: Time Limit Exceeded)

package main

import (
	"fmt"
)

func sort(slice [][2]int) [][2]int {
	for i := 0; i < len(slice); i++ {
		minIndex := i
		for j := i; j < len(slice); j++ {
			if slice[minIndex][0] > slice[j][0] {
				minIndex = j
			}
		}
		tmp := slice[i]
		slice[i] = slice[minIndex]
		slice[minIndex] = tmp
	}
	return slice
}

func main() {
	var N, M int
	fmt.Scan(&N, &M)
	val := make([][2]int, N)
	for i := 0; i < N; i++ {
		fmt.Scan(&(val[i][0]), &(val[i][1]))
	}
	val = sort(val)
	res := 0
	for _, v := range val {
		if M >= v[1] {
			M -= v[1]
			res += v[1] * v[0]
		} else {
			res += M * v[0]
			break
		}
	}
	fmt.Println(res)
}