An airplane wants to fly from $A$ to city $B$ which is $1000 \text{ km}$ due north of city $A$. The speed of the plane in still air is $500\text{ km/hr}$. The pilot neglects the effect of the wind and directs his plane due north and two hours later find himself $300\text{ km}$ due north-east of city $B$. The magnitude of wind velocity is (in $\text{ km/hr}$) .