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.util.Arrays;
import java.util.Objects;
import java.util.Scanner;

public class Main {
    public static class Can implements Comparable<Can> {
        long a;
        long b;
        public Can() {
            this.a = 0;
            this.b = 0;
        }
        public Can(long a, long b) {
            this.a = a;
            this.b = b;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Can can = (Can) o;
            return a == can.a &&
                    b == can.b;
        }

        @Override
        public int hashCode() {
            return Objects.hash(a, b);
        }

        @Override
        public int compareTo(Can o) {
            if (this.a < o.a) {
                return -1;
            } else if (this.a > o.a) {
                return 1;
            } else {
                return 0;
            }
        }
    }
    public static void main(String [] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int m = scanner.nextInt();
        Can[] cans = new Can[n];
        for (int i = 0; i < n; ++i) {
            long a = scanner.nextLong();
            long b = scanner.nextLong();
            cans[i] = new Can(a, b);
        }
        Arrays.sort(cans);
        int buy = 0;
        long money = 0;
        int i = 0;
        while (buy < m) {
            int currentBuy = (int)Math.min(m - buy, cans[i].b);
            money += currentBuy * cans[i].a;
            buy += currentBuy;
            ++i;
        }
        System.out.println(money);

    }

}