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 (1.14.1) (judge verdict: Accepted)

package main

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

var scanner = bufio.NewScanner(os.Stdin)

func init() {
	scanner.Split(bufio.ScanWords)
}

func ScanInt() (ret int) {
	var err error
	if scanner.Scan() {
		txt := scanner.Text()
		if ret, err = strconv.Atoi(txt); err != nil {
			_, _ = fmt.Fprintf(os.Stderr, err.Error())
		}
	}
	return
}

type Pair struct{ A, B int }

func main() {
	n, m := ScanInt(), ScanInt()

	pairs := make([]Pair, n)
	for i := 0; i < n; i++ {
		pairs[i].A, pairs[i].B = ScanInt(), ScanInt()
	}
	sort.Slice(pairs, func(i, j int) bool {
		return pairs[i].A < pairs[j].A
	})

	money := 0
	for i := 0; i < n; i++ {
		if m-pairs[i].B <= 0 {
			money += m * pairs[i].A
			break
		} else {
			money += pairs[i].A * pairs[i].B
			m -= pairs[i].B
		}
	}
	fmt.Println(money)
}