How to Create Calculator in Batch

Before creating a calculator lets go through a simple code tutorial..
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1>(echo) code "echo" are used to write a text for e.g " echo Welcome to Instructables".
2>(color 30)code "color" are used to give color to background and font.
3>(set /p =)code "set /p" is used to create console in program.
4>(title)code "title" is used to give title to program
5>(start)code "start" is used to start an external program.
6>(pause)code "pause" is used to pause a program to avoid, crash.
7>(%username%)code "%username%" just adds the name of the profile name of respective OS.
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"WIP" (Work-in-progress).

@echo off
title calculator
color 0A
:main
echo.
echo ---------------------------
echo.
echo WELCOME %USERNAME% to
echo.
echo Cal'C Ver 0.2
echo.
echo ---------------------------
echo.
echo /////////////////////////////////////
echo Your previous Calulated number was %sum%
echo ////////////////////////////////////
echo.
echo Enter the specified alphabet to continue..
echo.
echo a)Addition
echo.
echo b)Subtraction
echo.
echo c)Divison
echo.
echo d)Multipication
echo.
echo e)Square, Cube or any power (by Prof.Pickle)
echo.
set /p do=Value.
if %do%== a goto add
if %do%== A goto add
if %do%== b goto sub
if %do%== B goto sub
if %do%== c goto div
if %do%== C goto div
if %do%== d goto mul
if %do%== D goto mul
if %do%== e goto power1
if %do%== E goto power1
echo.
cls
echo Invalid value = %do%
echo.
pause
cls
goto main

:add
cls
echo ADDITON
echo.
set /p no1="num1. "
echo       +
set /p no2="num2. "
set /a sum=no1+no2
echo ------------
echo %sum%
echo.
pause
cls
goto main


:sub
cls
echo SUBTACTION
echo.
set /p no1="num1. "
echo       -
set /p no2="num2. "
set /a sum=no1-no2
echo ------------
echo %sum%
echo.
pause
cls
goto main


:div
cls
echo DIVISON
echo.
set /p no1="num1. "
echo       /
set /p no2="num2. "
set /a sum=no1/no2
echo ------------
echo %sum%
echo.
pause
cls
goto main


:mul
cls
echo MULTIPLICATION
echo.
set /p no1="num1. "
echo       *
set /p no2="num2. "
set /a sum=no1*no2
echo ------------
echo %sum%
echo.
pause
cls
goto main

:power1
cls
echo Square, Cube or any power (by Prof.Pickle)
echo.
echo Select the number
set /p num=
cls
echo Select the power
set /p pow=
set /a pow=%pow%+1
set ans=%num%
cls
:power2
set /a pow=%pow%-1
if %pow% LSS 1 goto next
set /a ans=%ans%*%num%
goto power2
:next
echo Your answer is %ans%
echo.
echo To open Prof.Pickle's instrutable's profile type 'I'. Type 'M' to goto main
set /p open=
if %open%== I start https://www.instructables.com/member/Prof.+Pickle/
if %open%== i start https://www.instructables.com/member/Prof.+Pickle/
if %open%== m goto main
if %open%== M goto main
goto main