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()