Gai's solution will work for integers. A better solution is to make sure one of the numbers you enter contains a .0 to make it a float. To follow his example, although it works in this case, you would change it to 1000/2.0.
As an example, entering 5/2 will give you a radius of 2.
Entering 5/2.0 will give you a radius of 2.5.
Todd