input: Write a function that takes a string containing a numerical expression and returns the result.
  def eval_expr(expr):
     return eval(expr)
  assert eval_expr("2 + 3 * 4") == 14
  assert eval_expr("10 / 2 - 3") == 2
  assert eval_expr("5 * (2 + 3)") == 25
  assert eval_expr("10 - 3 * 2") == 4
  assert eval_expr("8 / 2 * (2 + 2)") == 16
output: def eval_expr(expr):
     return eval(expr)