#include <bits/stdc++.h>
using namespace std;
 
int main() {
	// your code goes here
	string s;
	cin>>s;
	int k;
	cin>>k;
	int n=s.size();
	int max_len=1;
	int max_ind=0;
	vector<int>dp(n,1);
	for(int i=1;i<n;i++)
	{
		if(abs(s[i]-s[i-1])<=k)
		{
			dp[i]=dp[i-1]+1;
		}
		else
 
		{
			dp[i]=1;
		}
		if(dp[i]>max_len)
		{
			max_len=dp[i];
			max_ind=i;
		}
	}
	int start=max_ind-max_len+1;
	cout<<s.substr(start,max_len)<<endl;
 
	return 0;
}

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