Find one missing number from an unsorted array of consecutive numbers in C#.

FindMissingNumber.cs

public static int FindMissingNumber(IEnumerable<int> numbers) 
{
    int expectedSum = (numbers.Count() + 1)*(numbers.Count()+2)/2;
    int realSum = 0;

    foreach (var number in numbers)
    {
        realSum += number;    
    }

    return (expectedSum-realSum);
}