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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.Map;
import java.util.Scanner;
import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            // 07:00
            int N = scanner.nextInt();
            int M = scanner.nextInt();
            long[] A = new long[N];
            int[] B = new int[N];
            TreeMap<Long, Integer> map = new TreeMap<>();
            for (int i = 0; i < N; i++) {
                long cost = scanner.nextLong();
                int num = scanner.nextInt();
                map.compute(cost, (k,v) -> v == null? num : num + v);
            }
            long totalCost = 0;
            int bought = 0;
            for (final Map.Entry<Long, Integer> kv : map.entrySet()) {
                long cost = kv.getKey();
                int maxnum = kv.getValue();
                int buyCount = Math.min(maxnum, M - bought);
                totalCost += buyCount * cost;
                bought += buyCount;
                if (bought >= M) {
                    break;
                }
            }
            System.out.println(totalCost);
        }
    }
}