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.util.*;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int M = sc.nextInt();
    int A[] = new int[N];
    int B[] = new int[N];
    Map<Integer, Integer> mMap = new HashMap<Integer, Integer>();
    for (int i = 0; i < N; i++) {
      A[i] = sc.nextInt();
      B[i] = sc.nextInt();
      mMap.put(A[i], B[i]);
    }
    int sum = 0;
    int ans = 0;
    Object[] mapkey = mMap.keySet().toArray();
    Arrays.sort(mapkey);
    for (Integer nKey : mMap.keySet()) {
      if (mMap.get(nKey) <= M) {
        ans += nKey * mMap.get(nKey);
        M -= mMap.get(nKey);
      } else {
        ans += nKey * M;
        M -= M;
      }
      if (M == 0) {
        System.out.println(ans);
        return;
      }

    }
  }
}