import java.util.*;

public class Run {
    public static void main (String arg[]) {
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        long C,D;
        long count = 0;
        for (int i = 0; i < N; i++) {
            C = scan.nextLong();
            if (C%2 == 0) C /= 2;
            else C = C/2 + 1;
            D = scan.nextLong();
            count += C * D;
        }
        System.out.println(count);
    }
}