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: Time Limit Exceeded)

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();
		Long[] A = new Long[N];
		Integer[] B = new Integer[N];
		for (int i=0;i<N;i++) {
			A[i]=sc.nextLong();
			B[i]=sc.nextInt();
		}
		Long[] C = new Long[N];
		for (int i=0;i<N;i++) {
			C[i]=A[i];
		}
		Arrays.sort(A);
		List<Long> AA = Arrays.asList(A);
		List<Long> AAA = new ArrayList<Long>(AA);
		List<Integer> BB = Arrays.asList(B);
		List<Integer> BBB = new ArrayList<Integer>(BB);
		List<Long> CC = Arrays.asList(C);
		List<Long> CCC = new ArrayList<Long>(CC);
		List<Integer> D = new ArrayList<Integer>();
		for (int i=0;i<N;i++) {
			D.add(BBB.get(CCC.indexOf(AAA.get(i))));
			CCC.set(CCC.indexOf(AAA.get(i)),(long)-1);
		}
		int num=0;
		Long cost=(long)0;
		while (M>0) {
			if (M>=D.get(num)) {
				M=M-D.get(num);
				cost=cost+AAA.get(num)*D.get(num);
				num++;
			}else {
				break;
			}
		}
		if (M>0) {
			cost=cost+AAA.get(num)*M;
		}
		System.out.println(cost);
	}
}