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: Wrong Answer)

package main

import (
	"fmt"
	"sort"
)

type Int64Slice []int64

func (p Int64Slice) Len() int           { return len(p) }
func (p Int64Slice) Less(i, j int) bool { return p[i] < p[j] }
func (p Int64Slice) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }

func main() {
	var n, m int
	fmt.Scan(&n)
	fmt.Scan(&m)

	order := make(Int64Slice, 0, n)
	pmap := map[int64]int64{}
	for index := 0; index < n; index++ {
		var a, b int64
		fmt.Scan(&a)
		fmt.Scan(&b)

		pmap[a] = b
		order = append(order, a)
	}

	sort.Sort(order)

	var re int64
	var c int
	for _, item := range order {
		can := pmap[item]
		var index int64
		for index = 0; index < can; index++ {
			re += item
			c++
			if c == m {
				fmt.Printf("%#v\n", re)
				return
			}
		}
	}
}