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"
)

func main() {
	var n, m int
	s := wordScanner(100)
	scanInts(s, &n, &m)

	var b int
	a, set := make([]int, n), make(map[int]int, n)
	for i := 0; i < n; i++ {
		scanInts(s, &a[i], &b)
		set[a[i]] += b
	}

	sort.Ints(a)

	var ans int64
	for _, ai := range a {
		if set[ai] >= m {
			ans += int64(ai * m)
			break
		}
		ans += int64(set[ai] * ai)
		m -= set[ai]
	}

	fmt.Println(ans)
}

func wordScanner(n int) *bufio.Scanner {
	s := bufio.NewScanner(os.Stdin)
	s.Split(bufio.ScanWords)
	b := make([]byte, n)
	s.Buffer(b, n)
	return s
}

func scanInts(s *bufio.Scanner, vals ...*int) {
	for i := range vals {
		s.Scan()
		n, _ := strconv.Atoi(s.Text())
		*vals[i] = n
	}
}