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: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class Main {
	public static void main(String args[]) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		Solver solver = new Solver();
		solver.init();
		solver.readHead(in.readLine());
		for (; solver.hasNext();) {
			solver.readBody(in.readLine());
		}
		solver.solve();
	}
}

class Solver {
	int N;
	int cnt;

	public void init() {
		N = 0;
		cnt = 0;
	}

	int M;

	class Shop {
		long A;
		long B;

		public Shop(long a, long b) {
			super();
			A = a;
			B = b;
		}
	}

	ArrayList<Shop> Shops = new ArrayList<Shop>();

	public void readHead(String str) {
		String[] strArr = str.split(" ");
		N = Integer.parseInt(strArr[0]);
		// System.out.println(N);
		M = Integer.parseInt(strArr[1]);
	}

	public boolean hasNext() {
		return cnt < N;
	}

	public void readBody(String str) {
		String[] strArr = str.split(" ");
		long a = Long.parseLong(strArr[0]);
		long b = Long.parseLong(strArr[1]);
		Shops.add(new Shop(a, b));
		cnt++;
	}

	public void solve() {
		Collections.sort(Shops, new Comparator<Shop>() {
			@Override
			public int compare(Shop a, Shop b) {
				long d = a.A - b.A;
				if (d == 0)
					return 0;
				else if (d > 0)
					return 1;
				else
					return -1;
			}
		});
		long m = M;
		long ans = 0;
		for (Shop s : Shops) {
			// System.out.println(s.A + " " + s.B);
			int d;
			if (m > s.B) {
				ans += s.A * s.B;
				m -= s.B;
			} else {
				ans += s.A * m;
				m = 0;
			}
			if (m <= 0)
				break;
		}
		System.out.println(ans);
	}
}