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.Map;
import java.util.Scanner;
import java.util.TreeSet;

public class Main {

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);

		int N = in.nextInt();
		int M = in.nextInt();

		TreeSet<Pair<Long, Long>> ab = new TreeSet<>();
		for (int i = 0; i < N; i++) {
			ab.add(new Pair<Long, Long>(in.nextLong(), in.nextLong()));
		}

		long total = 0;
		for (Pair<Long, Long> pair : ab) {
			if (pair.value >= M) {
				total += pair.key * M;
				break;
			}
			total += pair.key * pair.value;
			M -= pair.value;
		}
		System.out.println(total);
	}

	private static class Pair<K extends Comparable<K>, V extends Comparable<V>>
			implements Map.Entry<K, V>, Comparable<Pair<K, V>> {

		private K key;
		private V value;

		public Pair(K key, V value) {
			this.key = key;
			this.value = value;
		}

		@Override
		public K getKey() {
			return key;
		}

		@Override
		public V getValue() {
			return value;
		}

		@Override
		public V setValue(V value) {
			V v = this.value;
			this.value = value;
			return v;
		}

		@Override
		public String toString() {
			return "[" + key.toString() + ", " + value.toString() + "]";
		}

		@Override
		public int compareTo(Pair<K, V> o) {
			int c1 = this.key.compareTo(o.key);
			if (c1 != 0) {
				return c1;
			}
			return this.value.compareTo(o.value);
		}
	}
}