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 Int64 []int64

func (a Int64) Len() int           { return len(a) }
func (a Int64) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a Int64) Less(i, j int) bool { return a[i] < a[j] }

func scanNums(len int) (nums []int64) {
	var num int64
	for i := 0; i < len; i++ {
		fmt.Scan(&num)
		nums = append(nums, num)
	}
	return
}

func main() {
	var n int
	var m int64
	
	fmt.Scan(&n)
	fmt.Scan(&m)
	x := make(map[int64]int64)
	for i := 0; i < n; i++ {
	    var a, b int64
	    fmt.Scan(&a)
		fmt.Scan(&b)
		x[a] += b
	}
	
	var keys Int64
	for k, _ := range x {
	    keys = append(keys, k)
	}
    sort.Sort(keys)

//  sort.Slice doesn't work on the version 1.6	
// 	sort.Slice(keys, func(i, j int) bool {
//         return keys[i] < keys[j]
//     })
	
	var c1, c2 int64 = 0, 0
	for _, k := range keys {
	    if x[k] + c1 < m {
	        c1 += x[k]
	        c2 += k * x[k]
	    } else {
	        c2 += k * (m - c1)
	        break
	    }
	}
    fmt.Println(c2)
}