How to correctly calculate the MSE?

Dear all,

I need to calculate MSE from my vectors (input = xx, output = Y). Is this correct? If so, how do I convert the resulting number is a real number.

This si my code:

load ('net_velky_tvar_nova');
for o = 1:1:101
  P = velky{1,o};
  xx = velky{2,o};
  Y = sim(net,P);
  MSE = sum((xx-Y).^2)/101;
  disp(MSE)
end

Thank you for your answers.