(defn r-squared [x y]
 (+ (* (- 0.5 x) (- 0.5 x))
    (* (- 0.5 y) (- 0.5 y))))

(defn calculate-pi [t]
  (loop [hits 0
         throws-left t] ; initial number of throws
    (cond ;kinda like if-elseif-else...
      (zero? throws-left)
        (* 4.0 (/ t hits)); all thrown, return value
      (< (r-squared (rand) (rand)) 0.25) ; not bothering with a let here.
        (recur (inc hits) (dec throws-left)) ; hit, inc hits
      :else
        (recur hits (dec throws-left))))) ; no hit, restart, but don't inc hits

(println (calculate-pi 10))