A and B terms are wrong.
What you wrote is the square of sum of differences. Instead formula is the sum of square of differences.
The code should be written as follow:
A = sum((x-y).^2);
B = sum((x-(mean(x))).^2);
fraction = (A/B);
NS = 1-fraction;
Pay attention also to space left between "sum" and "(x-y)" in your code: it is not correct.