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 (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
	"strings"
)

func main() {
	var (
		In        = ReadLineInt()
		ShopCount = In[0]
		Shops     = map[int]int{}
		Prices    = make([]int, 0)
		Target    = In[1]
	)

	for i := 0; i < ShopCount; i++ {
		Input := ReadLineInt()
		Prices = append(Prices, Input[0])
		Shops[Input[0]] = Input[1]
	}

	sort.Ints(Prices)

	Result := 0
	for i := 0; i < len(Prices); i++ {
		Price := Prices[i]
		BuyLimit := Shops[Price]

		if BuyLimit >= Target {
			Result += Price * Target
			break
		}

		Result += Price * BuyLimit
		Target -= BuyLimit
	}

	fmt.Println(Result)
}

/////////////////////////////////////////

func fln(format string, elements ...interface{}) {
	fmt.Printf(format+"\n", elements...)
}

var BufReader = bufio.NewReaderSize(os.Stdin, 1e6)

func ReadLine() string {
	a, _, _ := BufReader.ReadLine()
	return string(a)
}

func ReadLineInt() (result []int) {
	in := strings.Split(ReadLine(), " ")
	result = make([]int, 0, 2)
	for _, v := range in {
		t, _ := strconv.Atoi(v)
		result = append(result, t)
	}
	return
}