using System;
using System.Linq;

class No146
{
    static void Main()
    {
        var count = Int64.Parse(Console.ReadLine());
        long ans = 0;
        for (long i = 0; i < count; ++i)
        {
            var chair = Console.ReadLine().Split(' ').Select(x => Int64.Parse(x)).ToArray();
            var p = chair[0] % 2 == 0 ? chair[0] / 2 : chair[0] / 2 + 1;
            ans += p * chair[1];
        }
        Console.WriteLine(ans);
    }
}