a = "1234345"
b = "1239"
c = 0
r = []
max = a.length > b.length ? a.length : b.length

(1..max).each{ |i|
  n = a[-i].to_i + b[-i].to_i + c
  p n
  if n < 10 then
    c = 0
    r.unshift n
  else
    r.unshift n % 10
    c = n / 10
  end

  }

  p r.join