13. (5 points)
A forest witch has a 1-liter medicine bottle, filled with medicine. In each operation, she pours out half of the medicine in the bottle, then adds 0.5 liters of water, and mixes it evenly. After at least $\qquad$ rounds of operations, the concentration of the medicine in the bottle will not exceed $\frac{1}{1000}$ of the initial concentration.
$10$