You can type math operations anywhere e.g. notepad , then select them and then press the hotkey to calculate the result. The default hotkey is Control + C, Control + D.
The first time the application is being run the settings window is being shown. There you can enter in the textbox the second key for the hotkey i.e. by default it is the key D. By default the first key combination is Control - C and this cannot change.
For example, if you enter E in the textbox the hotkey will become Control + C, Control + E.
To run the application at Windows startup hidden just check or leavve checked the option "Run at Windows Startup" in the "Options" menu.
To copy the result of the math calculation but not paste it, check the "Copy Result to Clipboard" option.
To minimize the application to the windows system tray leave checked the option "Minimize to System Tray".
To open the setings window right click on the Window System Tray icon and select "Open" from the menu.