#include<bits/stdc++.h>
#include<atcoder/all>
#include<functional>

using namespace std;
using ll = long long;

int main() {
    ll N, K; cin >> N >> K;
    string t; cin >> t;
    
    // DP[i番目] := i〜endのパターン数
    vector<ll> DP(N, -1);
    function<ll(int)> dfs = [&](int n) {
        if (-1<DP[n]) return DP[n];
        if (N-2<=n) {
            DP[n] = 1;    
            return 1LL;
        }
        string s2 = t.substr(n, 2); 
        int d2 = stoi(s2);
        ll res = 0LL;
        if (d2<=26) {
            res += dfs(n+1);
        }
        res += dfs(n+2);
        return DP[n] = res;
    };
    dfs(0);

    string res = "";
    ll count = 0;
    for (int i=0;i<N;) {
        string s2 = t.substr(i, 2);
        int d1 = stoi(t.substr(i, 1));
        int d2 = stoi(s2);
        if (d2<=26) {
            if (count+DP[i+1]<K) {
                res += 'a' + (d2-1);
                count += DP[i+1];
                i +=2;
            }
            else {
                res += 'a' + (d1-1);
                i++;
            }
        }
        else {
            res += 'a' + (d1-1);
            i++;
        }
    }
    cout << res << endl;
}