using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var discount1 = new Discount

        {

            Amount = 10,

            ProductIds = new List<long>() { 100, 103, 104, 105 }

        };

        var discount2 = new Discount

        {

            Amount = 15,

            ProductIds = new List<long>() { 100, 102 , 103, 104 }

        };

        var discounts = new List<Discount>() { discount1, discount2 } ;

        var productDiscountDictionary = discounts

            .SelectMany(d => d

                .ProductIds

                .Select(p => new 

                { 

                    ProductId = p,

                    Amount = d.Amount

                }))

            .GroupBy(p => p.ProductId)

            .ToDictionary(

                p => p.Key,

                p => p.Max(d => d.Amount));

        foreach(var productDiscountKeyValue in productDiscountDictionary)

        {

            Console.WriteLine($"Product {productDiscountKeyValue.Key}, Amount: {productDiscountKeyValue.Value}");

        }

    }

    public class Discount

    {

        public decimal Amount {get ; set;}

        public List<long> ProductIds {get; set;}

    }

}