import math

total_length = 0
count = 0
for element_id in mw.selected_elements():
  if mw.element_shape(element_id) == "line2":
    node1 = mw.node(mw.nodes(element_id)[0])
    node2 = mw.node(mw.nodes(element_id)[1])
    length = math.sqrt((node1.x - node2.x)**2
                     + (node1.y - node2.y)**2
                     + (node1.z - node2.z)**2)
    total_length += length
    count += 1

if count == 0:
  mw.message("Select some line2 elements first")
else:
  mw.message("Length of " + str(count) + " line2 elements is " + str(total_length) + " m")