def func(): while True: try: s, n = str(input().strip()), int(input()) max_r = 0 max_s = '' for i in range(len(s)-n+1): sub = s[i:i+n] r = (sub.count('G')+sub.count('C'))/n if r > max_r: max_r = r max_s = sub print(max_s) except: break if __name__ == '__main__': func()