do while lines()
  i = 0
  line = linein()
  do while line\=''
    parse var line s.i line
    if datatype(s.i,'N') then  i = i+1
    else  do
      if 1<length(s.i) | 0=pos(s.i,'+-*/') then
        do; i = 2; leave; end
      i = i-1
      interpret 's.'||(i-1)'=s.'||(i-1) value('s.'||(i+1)) 's.'i
    end
  end
  if i=1 then  say value('s.'||(i-1))
         else  if i>1 then  say 'error'
end