; potholes

(define (potholes str)
  (let ((len (string-length str)))
    (let loop ((i 0) (fixes 0))
      (cond ((<= len i) fixes)
            ((char=? (string-ref str i) #\X)
              (loop (+ i 3) (+ fixes 1)))
            (else (loop (+ i 1) fixes))))))

(display (potholes ".X.")) (newline)
(display (potholes ".X...X")) (newline)
(display (potholes "XXX.XXXX")) (newline)
(display (potholes ".X.XX.XX.X")) (newline)