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 sc = bufio.NewScanner(os.Stdin)

func main() {
	scanInit()

	n := nextInt()
	m := nextInt()
	p := make([]pair, n)

	for i := 0; i < n; i++ {
		p[i].a = nextInt()
		p[i].b = nextInt()
	}

	sortPairs(p)

	cost := 0
	nowM := 0
	for i := 0; i < n; i++ {
		if nowM >= m {
			break
		}

		buy := min(p[i].b, m-nowM)
		cost += p[i].a * buy
		nowM += buy
	}
	fmt.Println(cost)

}
func min(a, b int) int {
	if a > b {
		return b
	}
	return a
}

// Pair ソート
// s := make([]pair, n)
// sortPairs(s)
type pair struct { // f=first, s=second
	a int
	b int
}

// []pair をf->sの順でソートする
func sortPairs(s []pair) {
	sort.Slice(s, func(i, j int) bool {
		if s[i].a == s[j].a { // fが同じ場合はsを比較する
			return s[i].b < s[j].b
		}
		return s[i].a < s[j].a
	})
	return
}

// ---- readfunc
func scanInit() {
	const cap = 200 * 1024 // Default=64
	var buf = make([]byte, cap)
	sc.Buffer(buf, cap)
	sc.Split(bufio.ScanWords)
	return
}
func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}
func nextStr() string {
	sc.Scan()
	return sc.Text()
}