Getting your values

-- All credits go to the author, Engineer
term.clear()
term.setCursorPos(1,1)

local buttons = { "left top", "right top", "left bottom", "right bottom" }
local cButton = { }

function display( input, button )
    term.setBackgroundColor( colors.black )
    term.setCursorPos(1,19)
    term.clearLine()
    if button then
        write( input )
    else
        write( "Current button: ".. input )
    end
end

print( "Click everywhere on the screen to place a marker." )
for i = 1, 3 do
    display( buttons[i] )
    while true do
        local evt, button, x, y = os.pullEvent( "mouse_click" )
        if buttons[i] == "left top" then
            paintutils.drawPixel( x, y, colors.orange )
            table.insert( cButton, x )
            table.insert( cButton, y )
            break
        elseif buttons[i] == "right top" and x > cButton[1] and y == cButton[2] then
            paintutils.drawPixel( x, y, colors.magenta )
            table.insert( cButton, x )
            break
        elseif buttons[i] == "left bottom" and y > cButton[2] and x == cButton[1] then
            paintutils.drawPixel( x, y, colors.lightBlue )
            table.insert( cButton, y )
            paintutils.drawPixel( cButton[3], cButton[4], colors.yellow )
            display( buttons[4] )
            break
        end
    end
end

sleep(1)
term.setBackgroundColor( 32768 )
term.clear()
term.setCursorPos( 1,1 )
print( "Your button will be printed shortly" )
sleep(1.5)
term.clear()
paintutils.drawLine( tonumber( cButton[1] ), tonumber( cButton[2] ), tonumber( cButton[3] ), tonumber( cButton[2] ), 512 )
paintutils.drawLine( tonumber( cButton[1] ), tonumber( cButton[4] ), tonumber( cButton[3] ), tonumber( cButton[4] ), 512 )
paintutils.drawLine( tonumber( cButton[1] ), tonumber( cButton[2] ), tonumber( cButton[1] ), tonumber( cButton[4] ), 512 )
paintutils.drawLine( tonumber( cButton[3] ), tonumber( cButton[2] ), tonumber( cButton[3] ), tonumber( cButton[4] ), 512 )
display( "Hit enter to continue ", true )

while true do
    local evt, key = os.pullEvent( "key" )
    if key == 28 then
        break
    end
end

term.clear()
term.setCursorPos( 1,1 )

print( "Your values\n" )

print( "xMin = " .. cButton[1] )
print( "xMax = " .. cButton[3] )
print( "yMin = " .. cButton[2] )
print( "yMax = " .. cButton[4] )