Let the original number of diamonds be \( x \). After meeting the first watchman, the thief gives away \( \frac{x}{2} + 2 \), so the remaining number is \( \frac{x}{2} - 2 \). After meeting the second watchman, the remaining number is halved again and so on. After the last meeting, he has one diamond left. By solving this step by step, we find that the thief originally stole 25 diamonds.