RomanNumerals Convert to int

RomanNumerals.cs

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

namespace Pairing
{
    public static class RomanNumerals
    {
        // C# specification explicitly forbids explicit operator calls.
        // So build our own.
        private static int Add(int left, int right)
        {
            return left + right;
        }

        private static int Subtract(int left, int right)
        {
            return left - right;
        }

        private static int Convert(char romanNumeral)
        {
            switch (romanNumeral)
            {
                case 'I': return 1;
                case 'V': return 5;
                case 'X': return 10;
                case 'L': return 50;
                case 'C': return 100;
                case 'D': return 500;
                case 'M': return 1000;
                default: return 0;
            }
        }
        private static Func<int, int, int> DetermineAdditiveOrSubtractiveNotation(char left, char right)
        {
            const string numerals = "IVXLCDM";
            var rightDistance = numerals.IndexOf(right) - numerals.IndexOf(left);

            if (rightDistance == 1 || rightDistance == 2)
            {
                return Subtract;
            }
            else
            {
                return Add;
            }
        }

        private static (char, int) Sum((char, int) aggregate, char leftNumeral)
        {
            var (rightNumeral, total) = aggregate;
            var value = Convert(leftNumeral);
            var combine = DetermineAdditiveOrSubtractiveNotation(leftNumeral, rightNumeral);
            var newTotal = combine(total, value);
            return (leftNumeral, newTotal);
        }

        private static TAccumulator RightFold<T, TAccumulator>(this IEnumerable<T> source, TAccumulator seed, Func<TAccumulator, T, TAccumulator> func)
        {
            return source.Reverse().Aggregate(seed, func);
        }

        public static int Convert(string input)
        {
            var (_, result) = input.ToCharArray()
                .RightFold((' ', 0), Sum);

            return result;
        }
    }
}