import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    long ans = 0;
    for(int i = 0; i < N; i++) {
      long c = sc.nextLong();
      long d = sc.nextLong();
      ans += (((c + 1) / 2) * d);
    }
    System.out.println(ans);
  }
}