Norman window optimization????ASAP HELP?

I am doing a norman window optimization problem. The perimeter total is 50. the equation is (2r*((50-2r-(2rpi)/2)/2))+((pir^2)/2). All i need is the largest r in the domain of that equation. If i get that i can calculate the max. I have the smallest r=0. I just need the largest possible value of r.