int[] numbers = new int[0];

int number;
string sum = "sum";
string exit = "exit";
string userInput;
bool isWorking = true;

while (isWorking)
{
    userInput = Console.ReadLine();

    if (userInput == sum)
    {
        int sumNumbers = 0;

        for (int i = 0; i < numbers.Length; i++)
        {
            sumNumbers += numbers[i];
        }

        Console.WriteLine(sumNumbers);
    }
    else if (userInput == exit)
    {
        isWorking = false;
    }
    else
    {
        int[] numbersClone = new int[numbers.Length + 1];

        for (int i = 0; i < numbers.Length; i++)
        {
            numbersClone[i] = numbers[i];
        }

        number = Convert.ToInt32(userInput);
        numbersClone[numbersClone.Length - 1] = number;
        numbers = numbersClone;
        Console.Clear();
    }
}