#include <bits/stdc++.h>
using namespace std;
#define int              long long int
#define double           long double
#define print(a)         for(auto x : a) cout << x << " "; cout << endl


const int M = 1000000007;
const int N = 3e5+9;
const int INF = 2e9+1;
const int LINF = 2000000000000000001;

inline int power(int a, int b, int mod=M) {
    int x = 1;
    a %= mod;
    while (b) {
        if (b & 1) x = (x * a) % mod; 
        a = (a * a) % mod;
        b >>= 1;
    }
    return x;
}


//_ ***************************** START Below *******************************




string a;

string consistency(int n, int k){

	vector<int> dp(n);
	dp[0] = 1;
	int maxi = 1;
	
	for(int i=2; i<n; i++){
		if( abs(a[i] - a[i-1]) <= k )  dp[i] = dp[i-1] + 1;
		else dp[i] = 1;
		
		maxi = max(maxi, dp[i]);
	}
	
	int s = 0, e = 0;
	while(e<n){
		
		if( abs(a[e] - a[s]) > k ){
			s = e;
		}
		
		if(e-s+1 < maxi){
			e++;
		}
		else{
			return a.substr(s, maxi);
		}
	}
	
	return "-1";
}















int practice(int n){


    return 0;
}





void solve() {
    
    int n, k;
    cin >> n >> k;
    
	cin >> a;
	
    cout << consistency(n, k) << endl;


}





int32_t main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    int t = 1;
    // cin >> t;
    while (t--) {
        solve();
    }

    return 0;
}