import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		List<String> input = new ArrayList<>();

		while (sc.hasNext()) {
			input.add(sc.nextLine());
		}

		System.out.println(solve(input));

		sc.close();
	}

	public static int solve(List<String> input) {
		int t = 0;
		input.remove(0);
		for (int i = 0; i < input.size(); i++) {
			int s = Integer.valueOf(input.get(i).split(" ")[0]);
			int k = Integer.valueOf(input.get(i).split(" ")[1]);
			if (s % 2 == 0) {
				t += (s / 2) * k;
			} else {
				t += (s + 1) / 2 * k;
			}
		}
		return t;
	}
}