题解 | #DNA序列#

# while True:
#     try:
#         DNA=input().strip()
#         n=int(input())
#         max_counts=0
#         max_DNA=[]
#         for i in range(len(DNA)):
#             GC_counts=DNA.count('G',i,i+n)+DNA.count('C',i,i+n)
#             if GC_counts>max_counts:
#                 max_counts=GC_counts
#                 max_DNA=DNA[i:i+n]
#         print(max_DNA)
#     except:
#         break

while True:
    try:
        DNA = input().strip()
        n = int(input())
        max_DNA = []
        GC_Ratio = []
        for i in range(len(DNA)):
            GC_counts = DNA[i:i+n].count('G') + DNA[i:i+n].count('C')
            max_DNA.append(DNA[i:i+n])
            GC_Ratio.append(GC_counts/n)
        m = max(GC_Ratio)
        print(max_DNA[GC_Ratio.index(m)])
    except:
        break