tes=input()
n=int(input())
c=0
t=''
for i in range(len(tes)-n+1):
    a=tes[i:i+n]
    b=0
    for s in range(n):
        if a[s]=='G' or a[s]=='C':
            b+=1
    if b>c:
        c=b
        t=a
print(t)