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: Accepted)

package main

import (
	"fmt"
	"sort"
)

type shop struct {
	a, b int
}

type byPrice []shop

func (a byPrice) Len() int           { return len(a) }
func (a byPrice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a byPrice) Less(i, j int) bool { return a[i].a < a[j].a }

func main() {
	var n, m int
	fmt.Scan(&n, &m)
	s := make([]shop, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&s[i].a, &s[i].b)
	}

	sort.Sort(byPrice(s))

	c := 0
	for i := 0; i < n; i++ {
		if s[i].b >= m {
			c += s[i].a * m
			fmt.Println(c)
			return
		}
		m -= s[i].b
		c += s[i].a * s[i].b
	}
}