Zero calculator

Some useful features:
1. The calculating display as an expression
2. Touch = then ans to get final result to continue to calculate
3. Correct each number in the expression or back delete
4. View expression in vertical
5. In calculating history user can open to continue edit or view expression in vertical
6. Edit label for each number when calculating or view expression in vertical
7. Hold 0 button to choose thousand, million or billion
8. Hold Backspace button to delete number
9. External keyboard support:
* Support Number pad
* Enter for =
* ESC for AC
* Delete, Backspace for correct
* Shift + Delete or Shift + Backspace for delete number
* k, m, b for enter Thousand, Million, Billion
* x for Multiplation (*)
10. Paste the number, copy the result in the text: touch or hold the label showing the number entered
11. Copy expression in vertical in image to share