input: Write a python function to return a list where each element is the sum of consecutive elements from the input list.
  assert sum_pairs([2, 4, 5, 10]) == [6, 9, 15]
  assert sum_pairs([1, 2, 3, 4]) == [3, 5, 7]
  assert sum_pairs([10, 20, 30, 40, 50]) == [30, 50, 70, 90]
output: def sum_pairs(lst):
      return [lst[i] + lst[i+1] for i in range(len(lst)-1)]