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 Java7 (OpenJDK 1.7.0) (judge verdict: Wrong Answer)

import java.util.Scanner;
import java.util.Arrays;
 
class Main {
  public static void main(String args[]) {
        Scanner scanner1 = new Scanner(System.in);
       
        int N = scanner1.nextInt();
        int M = scanner1.nextInt();
        
        int A[] = new int[N];
        int B[] = new int[N];
        
        int As[] = new int[N];
        int Bs[] = new int[N];
        
        for(int i = 0;i < N;i++){
        	A[i] = scanner1.nextInt();
        	B[i] = scanner1.nextInt();
        	As[i] = A[i];
        	Bs[i] = B[i];
        }
        
        Arrays.sort(As);
        for(int i = 0;i < N;i++){
        	for(int j = 0;j < N;j++){
        		if(As[i] == A[j]){
        			Bs[i] = B[j];
        		}
        	}
        }
              
        int sumy = 0;
        int sumh = 0;
        int n = 0;
        for(int i = 0;i < N;i++){
        	sumh = sumh + Bs[i];
        	if(sumh >= M){
        		sumy = sumy + As[i]*(M-(sumh-Bs[i]));
        		break;
        	}
        	sumy = sumy + As[i]*Bs[i];
        }
        
        
        System.out.println(sumy);
  }
}