Find Missing Number recursive

using System;
using System.Collections.Generic;

class Program
{
    /// <summary>
    /// Find missing number in ordered list of integers
    /// </summary>
    /// <param name="list">Integer list</param>
    /// <returns>Missing number</returns>
    public static int FindMissingNumber(IList<int> list)
    {
        if (list == null) throw new ArgumentNullException("list");
        return FindMissingNumber(list, 0, list.Count);
    }

    /// <summary>
    /// Recursively find missing missing number in ordered list
    /// </summary>
    /// <param name="a">Lower bound</param>
    /// <param name="b">Upper bound</param>
    /// <returns>Missing number</returns>
    private static int FindMissingNumber(IList<int> list, int a, int b)
    {
        // Find midpoint (m)
        var m = a + (b - a) / 2;
        if (a == b) return m + 1;
        if (list[m] == m) return m;
        if (list[m] == m + 1) return FindMissingNumber(list, m + 1, b);
        return FindMissingNumber(list, a, m);
    }

    static void Main(string[] args)
    {
        Action<IList<int>> Time = list =>
        {
            var timer = System.Diagnostics.Stopwatch.StartNew();
            var result = FindMissingNumber(list);
            timer.Stop();
            Console.WriteLine(
                "Result {0} Elapsed: {1}ms",
                    result,
                    timer.ElapsedMilliseconds);
        };

        Time(new[] {2,3,4});
        Time(new[] {1,2,3});
        Time(new[] {1,3,4});
        var tenMillion = new int[10000000];
        for(int i = 0; i < tenMillion.Length; i++) tenMillion[i]= i+1;
        Time(tenMillion);
        return;
    }
}