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("5-2") == 3
  assert eval_expr("10/2") == 5
  assert eval_expr("3**2") == 9
  assert eval_expr("7*8+1") == 57
output: def eval_expr(expr):
     return eval(expr)