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((String[])input.toArray()));

		sc.close();
	}

	public static int solve(String[] in) {
		int N = Integer.valueOf(in[0]);
		int[][] cc = new int[N][];
		for (int i = 1; i < N + 1; i++) {
			cc[i - 1] = new int[2];
			cc[i - 1][0] = Integer.valueOf(in[i].split(" ")[0]);
			cc[i - 1][1] = Integer.valueOf(in[i].split(" ")[1]);
		}

		int total = 0;
		for (int i = 0; i < N; i++) {
			if (cc[i][0] % 2 == 0) {
				total += (cc[i][0] / 2) * cc[i][1];
			} else {
				total += (cc[i][0] + 1) / 2 * cc[i][1];
			}
		}
		return total;
	}
}