Pi is the ratio of a circle's circumference to it's diameter:
pi = C/d
Since the length of the diameter is twice that of the radius (d = 2r), we get:
pi = C/2r
To get the first 50 decimals of pi, use Wolfram Alpha.
How about 3.14159265 35897932384626433832795028841971693993751 to be exact.