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*5")==17
  assert eval_expr("5-2")==3
  assert eval_expr("10/2+3")==8
  assert eval_expr("4*5-2")==18
  assert eval_expr("3**2")==9
output: def eval_expr(expr):
     return eval(expr)