using System;

public class Program

{

    public static void Main()

    {

        double imageWidth = 1920;

        double imageHeight = 1080;

        double aspectRatio = imageWidth / imageHeight;

        double newImageWidth = 0;

        double newImageHeight = 0;

        double minWidth = 640;

        double minHeight = 640;

        double diffWidth = imageWidth- minWidth;

        double diffHeight = imageHeight - minHeight;

        if (diffWidth > 0 && diffHeight > 0) 

        {           

            if (diffWidth < diffHeight) 

            {

                newImageWidth = Math.Round(minHeight * aspectRatio);

                newImageHeight = minHeight;

                if (newImageWidth < minWidth) 

                {

                    newImageWidth = minWidth;

                    newImageHeight = Math.Round(minWidth * (1/aspectRatio));

                }

            }

            else

            {

                newImageWidth = minWidth;

                newImageHeight = Math.Round(minWidth * (1/aspectRatio));

                if (newImageHeight < minHeight) 

                {

                    newImageWidth = Math.Round(minHeight * aspectRatio);

                    newImageHeight = minHeight;

                }

            }

        }

        else 

        {

            newImageWidth = imageWidth;

            newImageHeight = imageHeight;

        }

        double newAspectRatio = newImageWidth / newImageHeight;

        Console.WriteLine("Old: {0}/{1} = {2}, new: {3}/{4} = {5}",

                          imageWidth, imageHeight, aspectRatio, newImageWidth, newImageHeight, newAspectRatio);

​

    }

}