internal interface ICompositePart
        {
            double Evaluate();
        }

        public class Addition : ICompositePart
        {
            public Addition(ICompositePart op1, ICompositePart op2)
            {
                Op1 = op1;
                Op2 = op2;
            }

            public ICompositePart Op1 { get; }
            public ICompositePart Op2 { get; }

            public double Evaluate()
            {
                return Op1.Evaluate() + Op2.Evaluate();
            }
        }
        public class Multiplication : ICompositePart
        {
            public Multiplication(ICompositePart op1, ICompositePart op2)
            {
                Op1 = op1;
                Op2 = op2;
            }

            public ICompositePart Op1 { get; }
            public ICompositePart Op2 { get; }

            public double Evaluate()
            {
                return Op1.Evaluate() * Op2.Evaluate();
            }
        }
        public class Number : ICompositePart
        {
            public Number(int value)
            {
                Value = value;
            }

            public int Value { get; }

            public double Evaluate()
            {
                return Value;
            }
        }