import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
        int n = koko.nextInt();
       long[] c = new long[n];
       long[] d = new long[n];
       for(int i = 0; i<n; i++){
           c[i] = koko.nextLong();
           d[i] = koko.nextLong();
       }
       long total = 0;
       long[] t = new long[n];
       for(int i =0; i<n; i++){
           if(c[i]%2==0){
               t[i] = c[i]*d[i]/2;
               total = total + t[i];
           }else{
               t[i] = (c[i]+1)*d[i]/2;
               total = total + t[i];
           }
       }
       System.out.println(total);
    }
}