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: Wrong Answer)

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.*;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter out = new PrintWriter(System.out);
        String[] arr = br.readLine().split(" ");
        int N = Integer.parseInt(arr[0]);
        int M = Integer.parseInt(arr[1]);
        
        long ans = 0;
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < N; i++) {
            String[] a = br.readLine().split(" ");
            int A = Integer.parseInt(a[0]);
            int B = Integer.parseInt(a[1]);
            map.merge(A, B, (old, add) -> old + add);
        }

        Map<Integer, Integer> sortedMap = map.entrySet().stream()
            .sorted(Map.Entry.comparingByKey())
            .collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));
        
        long quantity = 0;
        for (Map.Entry<Integer, Integer> entry: sortedMap.entrySet()) {
            long p = entry.getKey();
            long q = entry.getValue();
            if (quantity + q < M) {
                quantity += q;
                ans += p * q;
            } else {
                while (quantity < M) {
                    quantity++;
                    ans += p;
                }
            }
        };
        out.println(ans);
        out.close();
    }
}