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: Runtime Error)

package main

import (
	"fmt"
)

var m int

func main() {
	var n, t int
	sum := 0
	fmt.Scan(&n, &m)
	a := make([]int, n)
	b := make([]int, n)

	for i := 0; i < n; i++ {
		fmt.Scan(&a[i], &b[i])
	}

	for a != nil {
		deside := decidemin(a)
		t, a, b = decount(a, b, deside, sum)
		sum = t
	}
	fmt.Println(sum)
}

func decidemin(a []int) int {
	var i int
	min := 0
	for i = 0; i < len(a)-1; i++ {
		if a[i] < min {
			min = a[i]
		}
	}
	return i
}

func decount(a []int, b []int, k int, sum int) (int, []int, []int) {
	if b[k] <= m {
		sum = sum + a[k]*b[k]
		m -= b[k]
		a = delete(a, k)
		b = delete(b, k)
		return sum, a, b
	} else {
		for m > 0 {
			sum += a[k]
			m = m - 1
		}
		return sum, nil, nil
	}
}

func delete(s []int, i int) []int {
	s = append(s[:i], s[i+1:]...)
	n := make([]int, len(s))
	copy(n, s)
	return n
}