3. In a district of a city, the streets form an $11 \times 11$ grid. A water truck starts from the bottom-left corner $A(0,0)$ every day and drives to the top-right corner $B(10,10)$. At each intersection, the driver randomly chooses the direction to proceed, as long as it does not take a longer route. One day, the street from $(9,9)$ to $(10,9)$ is blocked due to an accident, but the driver is unaware of this when starting. What is the probability that the water truck can still reach $B$ as usual?
3. $1-\frac{\mathrm{C}_{18}^{9}}{2^{19}}$.

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3. $1-\frac{\mathrm{C}_{18}^{9}}{2^{19}}$.