Yet another π formula

[EdS2]

I like this double factorial approach. Here it is in BBC Basic:

Code:

PRINT "Calculate Pi with double factorial"
L=21:N=1:D=1:S=1
FOR K=1 TO L
N=N*K
D=D*(2*K+1)
S=S+N/D
PRINT 2*(S+N/D)
NEXT