input: Write a function to read in 15 integers and return their sum.
  
  assert sum_integers([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13, 14])==105
  assert sum_integers([-1, -5, -9, 0, -2, -4, -7, -8, -6, -10, -3, -11, -12, -13, -14])==-95
  assert sum_integers([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])==0
  assert sum_integers([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15])==120
  assert sum_integers([-1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1])==0
output: def sum_integers(numbers):
      return sum(numbers)