s = input()
N = int(input())
ls = []
for i in range(0,len(s)):
    ls.append(s[i:i+N])
#print(ls)
lst = []
for i in ls:
    if len(i) == N:
        lst.append(i)
#print(lst)
ld = []
for i in lst:
    ld.append((i.count('G')+i.count('C'))/len(i))
print(lst[ld.index(max(ld))])