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

namespace ConsoleApp3
{
    class Program
    {
        static void Main(string[] args)
        {
            List<int> list = new List<int> { 30000, 30001, 30003, 30004 };

            var next = list[list
                .OrderBy(i => i)
                .Skip(1)
                .Zip(list, (curr, prev) => curr - prev)
                .Select((diff, index) => new { diff, index })
                .First(x => x.diff > 1).index] + 1;
        }
    }
}