Lua indexes in adding script doesn't work properly

I have this code in lua (sorry if its bad).

function splitIntoTable(inputstr,sep)
  local t = {}
  for str in string.gmatch(inputstr,"([^" .. sep .. "]+)") do
    table.insert(t,str)
  end
  return t
end

function displayList(table)
  for k, v in ipairs(table) do
    print(table[k])
  end
end

local tocalc = "57 + 38"

print("Inputted: " .. tocalc)
tocalc = "0 " .. tocalc
local workwith = splitIntoTable(tocalc," ")
local did = 0
local doing = 1
local lenOfWorkwith = 0

for k in pairs(workwith) do
  lenOfWorkwith = lenOfWorkwith + 1
end

repeat
  if workwith[doing] == "+" then
    did = did + workwith[doing - 1] + workwith[doing + 1]
  end
  doing = doing + 1
until doing > lenOfWorkwith

did = math.floor(did + 0.5)

print("Result: " .. did)

I know it's a bit inefficient, but I just need it usable right now. Basically, what its supposed to do is simply plus numbers. For example, I put in 57 + 38, it works fine and gives me the correct calculation, but as soon as I put in 3 numbers (for example, 57 + 38 + 40), it breaks down and doesn't give the correct answer.