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

class No693{
    static void Main(){
        var a=Int32.Parse(Console.ReadLine());
        var b=Console.ReadLine().Split(' ').Select(x=>Int32.Parse(x)).OrderBy(x=>x).ToList();
        var c=0;
        for(int i=0;i<b.Count;++i){
            if(b[i]>1){
                b[i+1]+=(b[i]-1);
                c=b[i]-1;
                b[i]=1;
            }
        }
        Console.WriteLine(c);
    }
}