I don't want to register on your website. Sorry This is C# 4.0

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

namespace Contest4 {
    class Program {
        static void Main(string[] args) {
            Console.WriteLine("Result = {0}", Contest4(2, 3, 5, 15));
        }
        static long Contest4(long a, long b, long c, long d) {
            var inputs = new[] { a, b, c };
            var result = new List<long>(inputs);
            var counter = 0L;
            var multiple = 0L;
            foreach (var input in inputs) {
                counter = 1L;
                while ((multiple = input * counter++) < d)
                    if (!result.Contains(multiple))
                        result.Add(multiple);
            }
            return result.Sum();
        }
    }
}

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