calculation with list in list

[gianniskampanakis]

hello. with this code i can calculate 1 + 2 + 3 = 6

l = [1,2,3,]
def suml(l):
  s= 0
  for i in l:
    s = s + i
  return s  

but what can i do if i want to calculate this list ( 1 + 2 +3 +( 1 + 2) = 9)

l = [1,2,3,[1,2]]