You can solve any equation including adding with this code:
@echo off
title Richie's Calculator 3.0
:main
echo Welcome to Richie's Calculator 3.0
echo Press any key to begin calculating...
pause>nul
echo Enter An Equation
echo Example: 1+1
set /p
set /a sum=%equation%
echo.
echo The Answer Is:
echo %sum%
echo.
echo Press any key to return to the main menu
pause>nul
cls
goto main