import java.util.*; import java.io.*; public class Main { public static void main(String[] args) throws IOException{ BufferedReader br=new BufferedReader(new InputStreamReader(System.in)); String str=br.readLine(); char[] chars=str.toCharArray(); int num=Integer.valueOf(br.readLine()); // dp[i]表示i->i+num的子串中包含的GC数量,dp[i]=dp[i-1]-char[i-1]=GC?1+char[i+num-1]=GC?1 int max=0,maxIndex=0; //计算0-num的GC数量 for(int i=0;i<num;i++){ if(chars[i]=='G'||chars[i]=='C'){ max++; } } int last=max; //依次计算后续子串的GC数量 for(int i=1;i<chars.length-num+1;i++){ if(chars[i-1]=='G'||chars[i-1]=='C'){ last--; } if(chars[i+num-1]=='G'||chars[i+num-1]=='C'){ last++; } if(last>max){ max=last; maxIndex=i; } } System.out.print(str.substring(maxIndex,maxIndex+num)); } }