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 (
	"bufio"
	"fmt"
	"os"
	"sort"
)

var sc = bufio.NewScanner(os.Stdin)

func r() (n int) {
	sc.Scan()
	for _, v := range sc.Bytes() {
		n = n*10 + int(v-48)
	}
	return
}

func main() {
	sc.Split(bufio.ScanWords)
	n, m := r(), r()

	ab := make(map[int]int, n)
	keys := make([]int, 0, n)
	for i := 0; i < n; i++ {
		a, b := r(), r()
		if _, ok := ab[a]; !ok {
			ab[a] = b
			keys = append(keys, a)
		} else {
			ab[a] += b
		}
	}
	sort.Ints(keys)

	var count int
	var ans int64
	for _, k := range keys {
		switch {
		case count+ab[k] < m:
			count += ab[k]
			ans += int64(k) * int64(ab[k])
		case count+ab[k] == m:
			ans += int64(k) * int64(ab[k])
			fmt.Println(ans)
			return
		case count+ab[k] > m:
			ans += int64(k) * int64(m-count)
			fmt.Println(ans)
			return
		}
	}
}