Codility Lesson 3-3 Time Complexity

PermMissingElem.cs

using System;
// you can also use other imports, for example:
// using System.Collections.Generic;

// you can write to stdout for debugging purposes, e.g.
// Console.WriteLine("this is a debug message");

class Solution {
    public int solution(int[] A) {
        double sum = 0;
		double length = (double)A.Length;

		for (int i = 0; i < length; i++)
		    sum += A[i];
		    
		var supposedSum = (length + 1) * ((double)(length + 2) / 2);
        
        return (int)(supposedSum - sum);
        // write your code in C# 6.0 with .NET 4.5 (Mono)
    }
}