; Calcule a*b^n mod m
(defun exp-mod (a b n m)
    (if (= n 0)
        (mod a m)
        (if (evenp n)
            (exp-mod a (mod (* b b) m) (/ n 2) m)
            (exp-mod (mod (* a b) m) b (1- n) m))))

(format t "~d~%" (exp-mod 1 (/ (+ (read) (read)) 2) (read) 1000000007))