Solution: Missing Number in a Sorted Array

using System;
public class Program
{
    /// <summary>
    /// Finds a missing number from an array that contains sorted numbers starting from 1.
    /// </summary>
    /// <param name="arr">Array of sorted integers.</param>
    /// <returns>The missing number in the sorted array.</returns>
    public static int MissingNumber(int[] arr)
    {
        int actualNumber = 1; // Grows in ascending order
        foreach (int i in arr) // Iterating the entire array
        {
            if (i != actualNumber) // If the number in the array is not equal to actual number
            {
                return actualNumber; // This is the missing number in the array
            }
            actualNumber = actualNumber + 1; // Incrementing the actual number by 1
        }
        return -1; // If there is no missing number
    }
    // Driver code to test above method
    public static void Main(string[] args)
    {
        Console.WriteLine(MissingNumber(new int[] { 1, 2, 4 }));
        Console.WriteLine(MissingNumber(new int[] { 1, 2, 3, 4, 6 }));
        Console.WriteLine(MissingNumber(new int[] { 2, 3, 4, 5, 6 }));
        Console.WriteLine(MissingNumber(new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }));
    }
}