tt

public interface IApplication
{
    IResponse Respond(IRequest request);
}

public interface IRequest
{
    string Method { get; }
    string Uri { get; }

tt

public long CalculateChecksum(long value)
{
	long sum = 0;
	for (int i = 0; value != 0; i++)
	{
		for (var j = (value % 10) * (2 - i % 2); j != 0; j /= 10)
		{
			sum += j % 10;
		}
		value /= 10;