(define pow (lambda (a b) (if (<= b 0) 1 (* a (pow a (- b 1))))))
(define atan (lambda (x)(begin
  (define n 0)
  (define a 0)
  (define loop (lambda (i)
    (if (< i 10)
      (begin
        (set! a (+ (* 2 i) 1))
        (set! n (+ n (/ (* (pow -1 i) (pow x a)) a)))
        (loop (+ i 1)))
      n)))
  (loop 0))))

(define pi (+ (* 16 (atan (/ 1 10))) (* 16 (atan (/ 5 51))) (* -4 (atan (/ 1 239)))))
(print pi)
