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.Comparator;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		//		new Main().solveA();
		//		new Main().solveB();
		new Main().solveC();
		// new Main().solveD();
	}

	private void solveA() {
		Scanner scanner = null;
		int numH = 0;
		int numW = 0;
		int h = 0;
		int w = 0;

		try {
			scanner = new Scanner(System.in);
			numH = scanner.nextInt();
			numW = scanner.nextInt();
			h = scanner.nextInt();
			w = scanner.nextInt();

			int total = numH * numW;
			int col = h * numW;
			int row = w * (numH - h);
			System.out.println(total - col - row);

		} finally {
			if (scanner != null) {
				scanner.close();
			}
		}
	}

	private void solveB() {
		Scanner scanner = null;
		int numN = 0;
		int numM = 0;
		int numC = 0;

		try {
			scanner = new Scanner(System.in);
			numN = scanner.nextInt();
			numM = scanner.nextInt();
			numC = scanner.nextInt();
			int[] bArray = new int[numM];
			int[][] aArray = new int[numN][numM];

			for (int i = 0; i < bArray.length; i++) {
				bArray[i] = scanner.nextInt();

			}
			for (int i = 0; i < aArray.length; i++) {
				for (int j = 0; j < aArray[i].length; j++) {
					aArray[i][j] = scanner.nextInt();
				}
			}
			int cnt = 0;

			for (int i = 0; i < aArray.length; i++) {
				int res = 0;
				for (int j = 0; j < aArray[i].length; j++) {
					res += (aArray[i][j] * bArray[j]);
				}
				if (res + numC > 0) {
					cnt++;
				}
			}

			System.out.println(cnt);

		} finally {
			if (scanner != null) {
				scanner.close();
			}
		}
	}

	public static class LComp implements Comparator<Long[]> {

		@Override
		public int compare(Long[] o1, Long[] o2) {
			return Long.compare(o1[0], o2[0]);
		}

	}

	private void solveC() {
		Scanner scanner = null;
		int numN = 0;
		long numM = 0;

		try {
			scanner = new Scanner(System.in);
			numN = scanner.nextInt();
			numM = scanner.nextLong();

			Long[][] wk = new Long[numN][2];

			for (int i = 0; i < numN; i++) {
				wk[i][0] = scanner.nextLong();
				wk[i][1] = scanner.nextLong();
			}

			Arrays.sort(wk, new LComp());

			long resNumM = numM;
			long res = 0;
			for (int i = 0; i < wk.length; i++) {
				if (resNumM <= 0) {
					break;
				}
				long key = wk[i][0];
				long value = wk[i][1];
				if (resNumM >= value) {
					res += (key * value);
					resNumM -= value;
				} else {
					res += (key * resNumM);
					resNumM = 0;
				}

			}

			System.out.println(res);

		} finally {
			if (scanner != null) {
				scanner.close();
			}
		}
	}

	private void solveD() {
		Scanner scanner = null;
		int numN = 0;
		int numK = 0;
		int numS = 0;

		try {
			scanner = new Scanner(System.in);
			numN = scanner.nextInt();

			System.out.println("");

		} finally {
			if (scanner != null) {
				scanner.close();
			}
		}
	}
}