MATLAB Simulink: IF statement logic

General question, if I have MATLAB code looking like this:

for x=0:500
   if x <= 250
      y = 2*x+1;
   else
      y = 3*x+2;
   end
end

How could I have the same logic using blocks/systems in Simulink? Whenever I try switches and If blocks, I get an error like "Input Ports (1) of __ are involved in the loop.

I can share my model if it is helpful. But if someone could show me how to put this code in terms of a Simulink system, I'm sure I could figure it out. FYI, if it isn't obvious, x is my input and y is my output.

Thank you!