You aren't getting a result because your program is still running. Think about the size of the numbers you are calculating there in that method. 11 iterations is a stretch on my machine, 20 is probably far outside the reach of a common PC. Try running 5 iterations and optimizing your algorithm before going higher again.
Aside from that, lejlot is correct, you should negate the sign, not just substract it from one iteration to the next.
Edit: For 10 iterations I get the following result:
Pi: 3.140598667726060313997433309223757530269291966682082323...
Since we all know from memory that Pi starts out as 3.14159265358979323846264...
that's not all that accurate yet.