using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var parents = new [] 

        {

            new Parent { Id = 1, Due = 1000},

                new Parent { Id = 2, Due = 3000},

                new Parent { Id = 3, Due = 5000},

                new Parent { Id = 4, Due = 6000}

        };

        var children = new []

        {

            new Child { Id = 1, Parent = 1, Paid=600},

                new Child { Id = 2, Parent = 1, Paid=400},

                new Child { Id = 3, Parent = 2, Paid=1000},

                new Child { Id = 4, Parent = 2, Paid=1000},

                new Child { Id = 5, Parent = 2, Paid=1000},

                new Child { Id = 6, Parent = 3, Paid=2000}

        };

        var dueParents = parents.Where(parent => parent.Due > children.Where(child => child.Parent == parent.Id).Sum(child => child.Paid));

        foreach (var due in dueParents)

        {

            Console.WriteLine(due.Id);

        }

    }

    public class Parent

    {

        public int Id{get;set;}

        public int Due{get;set;}

    }

        public class Child 

        {

            public int Id {get;set;}

            public int Parent {get;set;}

            public int Paid {get;set;}

        }

}