import java.util.Arrays;
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();
			}
		}
	}

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

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

			//			TreeMap<Long, Long> shopmap = new TreeMap<Long, Long>();
			//
			//			for (long i = 0; i < numN; i++) {
			//				shopmap.put(scanner.nextLong(), 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, (x, y) -> Long.compare(x[0], y[0]));

			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) {
					resNumM -= value;
					res += (key * value);
				} else {
					resNumM -= value;
					res += (key * (resNumM + value));

				}

			}
			//			for (Entry<Long, Long> entry : shopmap.entrySet()) {
			//				if (resNumM <= 0) {
			//					break;
			//				}
			//				long key = entry.getKey();
			//				long value = entry.getValue();
			//				if (resNumM >= value) {
			//					resNumM -= value;
			//					res += (key * value);
			//				} else {
			//					resNumM -= value;
			//					res += (key * (resNumM + value));
			//
			//				}
			//
			//			}
			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();
			}
		}
	}
}

Measured memory_bytes: 98116