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: Wrong Answer)

package main

import (
	"fmt"
	"sort"
)

func main() {
	// Code for C - Energy Drink Collector
	var N, M int
	fmt.Scanf("%d %d", &N, &M)

	A := make(map[int]int, N)
	B := make([]int, N)
	var a, b int
	for i := 0; i < N; i++ {
		fmt.Scanf("%d %d", &a, &b)
		A[a] += b
		B[i] = a
	}

	var count int
	var cost int
	sort.Sort(sort.IntSlice(B))
	//fmt.Println(A, B)

	for i := 0; i < len(B); i++ {
		//fmt.Println(count, cost, A[B[i]], B[i])

		if M >= count+A[B[i]] {
			count += A[B[i]]
			cost += A[B[i]] * B[i]
		} else {
			cost += (M - count) * B[i]
			count = M
		}

		if count == M {
			break
		}
	}

	fmt.Println(cost)
}