input: Write a function to evaluate a given string containing a numerical expression and return the result.
  
  def eval_expr(expr):
     return eval(expr)
  
  assert eval_expr("2+2") == 4
  assert eval_expr("5*3-1") == 14
  assert eval_expr("10/2+3") == 8
  assert eval_expr("3**2") == 9
  assert eval_expr("4-6") == -2
output: def eval_expr(expr):
      return eval(expr)