I currently use an app that allows me to enter four sides irrespective of the shape of the polygon and it provides me with an area. But, yes, the app has an interface which asks me to select the type of polygon that im wishing to find the area of. there are about 50 odd shapes of which the app provides areas. I could share the name of the app with you just for reference. The app is: Area Calculator by NSDev (Nihon System Developer Corp.)