using System;

​

// Implement the method below that finds a missing number in an ordered array of positive integers, that start from 1, e.g.:

// FindMissingNumber([1,2,3,5,6,7,8]) returns 4

// FindMissingNumber([2,3,4,5]) returns 1

// FindMissingNumber([1,2,3,4,5]) returns 6

// FindMissingNumber([1,2,3,4,5,6,7,8,9,10,11,12,14,15,16,17]) returns 13

public class Program

{

    public static int FindMissingNumber(int[] numbers)

    {

        int missingNumber = 0;

        for(int i = 0; i<numbers.Length; i++)

        {

            if(numbers[i] != i+1) {

                return numbers[i]-1;

            }

            else missingNumber = numbers.Length + 1;

        }

        return missingNumber;

    }

    // do not change the code below this line

    // this is to test the method above

    public static void Main()

    {

        // FindMissingNumber([1,2,3,5,6,7,8]) returns 4

        var s1 = new[] {1,2,3,5,6,7,8};

        Console.WriteLine(FindMissingNumber(s1));

        // FindMissingNumber([2,3,4,5]) returns 1

        var s2 = new[] {2,3,4,5};

        Console.WriteLine(FindMissingNumber(s2));       

        // FindMissingNumber([1,2,3,4,5]) returns 6

        var s3 = new[] {1,2,3,4,5};

        Console.WriteLine(FindMissingNumber(s3));               

        // FindMissingNumber([1,2,3,4,5,6,7,8,9,10,11,12,14,15,16,17]) returns 13

        var s4 = new[] {1,2,3,4,5,6,7,8,9,10,11,12,14,15,16,17};

        Console.WriteLine(FindMissingNumber(s4));               

    }

}