No need to hesitate,the more you spend, the more income you want.
Application which has strong memorizable function,web approached by url or by buttons,calculator,painting canvas with temporary save and restore functions and 6 colors,2 thick and thin lines.
You can plus or minus continuously, but multiply and dividing, it needs put = each time.
Divide 10/3 answer 3, this is an error of course,in this case you could calculate 100/3 answer 33, or 1000/3 answer 333. No decimally calculation is available.
The small vacant space on the calculator,or memory textfields screens could be used for black only writing if you wish.
18 memorizable textfields including Index are small, but is arranged like matrix for the convenience of later search,and one is for big size letter.
There is one red colored letter button and one red color textfield.They are for retracting the keyboard which appears very often when you touch textfields.
3 finger double touch function of iPhone will be very helpful for reading small pictures or letters, and remember it needs setting "ZOOM" of iPhone in advance.