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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)

import java.math.BigInteger;
import java.util.Scanner;
import java.util.TreeMap;

public class Main {

  public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);

    long n = sc.nextLong();
    long m = sc.nextLong();
    TreeMap<Long, Long> map = new TreeMap<Long, Long>();
    BigInteger ans = BigInteger.ZERO;
    long bought = 0;

    for (int i = 0; i < n; i++) {
      long a = sc.nextLong();
      long b = sc.nextLong();
      map.put(a, b);
    }

    for (long key : map.keySet()) {
      if ((m - bought) <= 0) {
        break;
      }
      ans = ans.add(BigInteger.valueOf(key * Math.min(m - bought, map.get(key))));
      bought += Math.min(m - bought, map.get(key));
    }

    System.out.println(ans);

  }
}