def delta( listToDelta ):
  result = []
  lastItem = listToDelta[0]
  for i, item in enumerate(listToDelta):
    if i == 0:
      result.append(item)
    else:
      result.append(item-lastItem)
    lastItem = item
  return result

l = [2, 4, 6, 9, 7]
print(l)
l = delta(l)
print(l)