using System;
using System.Linq;

class No146{
    static void Main(){
        var count=Int64.Parse(Console.ReadLine());
        var ans=0;
        for(long i=0;i<count;++i){
            var chair=Console.ReadLine().Split(' ').Select(x=>Int32.Parse(x)).ToArray();
            var p=chair[0]%2==0?chair[0]/2:chair[0]/2+1;
            ans+=p*chair[1];
        }
        Console.WriteLine(ans);
    }
}