#include <bits/stdc++.h>
using namespace std;
int main() {
int n, k;
cin>>n>>k;
string s;
cin>>s;
vector<int> dp(n, 1);
int maxLength=1, maxIndex=0;
// dp[1]=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]>maxLength){
maxLength=dp[i];
maxIndex=i;
}
}
int startIndex = maxIndex - maxLength + 1;
cout << s.substr(startIndex, maxLength) << endl;
return 0;
}
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