#include <iostream>
#include <vector>
#include <cmath>
using namespace std;

int main() {
    int n,k;
    cin>>n>>k;
    string s;
    cin>>s;
    vector<int> dp(n,1);
   
    int ml=1;
    int mi=0;
   
    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]>ml) {
            ml=dp[i];
            mi=i;
        }
    
    	
    }
    int si=mi-ml+1;
    cout<<s.substr(si,ml)<<endl;
   
    return 0;
}