using System;
using System.Collections.Generic;
using System.Linq;

class Magatro
{
    static void Main()
    {
        int N = int.Parse(Console.ReadLine());
        int[] C = new int[N];
        int[] D = new int[N];
        for(int i = 0; i < N; i++)
        {
            string[] s = Console.ReadLine().Split(' ');
            int w = int.Parse(s[0]);
            int q=w%2==0?w/2:w / 2+1;
            D[i] = int.Parse(s[1])*q;
        }
        Console.WriteLine(D.Sum());
    }
}