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.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedReader;
import java.util.StringTokenizer;
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.Map;
import java.util.LinkedHashMap;
import java.util.Map.Entry;
import java.util.stream.Collectors;
import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Arrays;
import static java.util.Comparator.*;
import java.util.Comparator;

public class Main {

    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastScanner fs = new FastScanner(inputStream);
        PrintWriter pw = new PrintWriter(outputStream);
        int N = fs.nextInt(), M = fs.nextInt();
        Long minPrice = 0L;
        Shop[] shops = new Shop[N];
        for(int i = 0; i < N; i++) shops[i] = new Shop(fs.nextLong(), fs.nextLong());
        Shop.sort(shops);
        for(int i = 0; i < shops.length && M > 0; i++) {
            if(M > shops[i].getAmount()) {
                minPrice += shops[i].getAllPrice();
                M -= shops[i].getAmount();
            } else {
                minPrice += shops[i].getPrice(M);
                break;
            }
        }
        pw.println(minPrice);
        pw.close();
    }

}

class Shop {
    
    private Long price;
    private Long amount;

    Shop() {
        price = 0L;
        amount = 0L;
    }
    
    Shop(Long price, Long amount) {
        this.price = price;
        this.amount = amount;
    }

    public Long getPrice() {
        return price;
    }
    
    public Long getPrice(int amount) {
        return price * amount;
    }
    
    public Long getAmount() {
        return amount;
    }
    
    public Long getAllPrice() {
        return price * amount;
    }
    
    public static Shop[] sort(Shop[] shops) {
        Arrays.sort(shops, Comparator.comparing(Shop::getPrice));
        return shops;
    }

}

class FastScanner {

    private BufferedReader reader = null;
    private StringTokenizer tokenizer = null;
 
    public FastScanner(InputStream in) {
        reader = new BufferedReader(new InputStreamReader(in));
        tokenizer = null;
    }
 
    public String next() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }
    public String nextLine() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                return reader.readLine();
            } catch (IOException e) {
                    throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken("\n");
    }
 
    public long nextLong() {
        return Long.parseLong(next());
    }
 
    public int nextInt() {
        return Integer.parseInt(next());
    }
 
    public double nextDouble() {
        return Double.parseDouble(next());
    }
 
    public int[] nextIntArray(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++) a[i] = nextInt();
        return a;
    }
 
    public long[] nextLongArray(int n) {
        long[] a = new long[n];
        for (int i = 0; i < n; i++) a[i] = nextLong();
        return a;
    }
 
}