#include <bits/stdc++.h>
#include <cstdlib>
#include <math.h>
using namespace std;

using ll = long long;


int main(){
    int n,k;
    cin >> n >> k;
    string s;
    cin >> s;
    vector<ll>p(n+1,0);
    p[n]=1;
    if(s.back()!='0')p[n-1]=1;
    for(int i=n-2;i>=0;i--){
        if(s[i]=='0')continue;
        p[i]=p[i+1];
        string u;
        u+=s[i];
        u+=s[i+1];
        if(stoi(u)<=26){
            p[i]+=p[i+2];
        }
        if(p[i]>(1LL<<60))p[i]=(1LL<<60);
    }
    string ans;
    k--;
    for(int i=0;i<n;i++){
        if(i==n-1){
            ans.push_back(s[i]-'1'+'a');
            continue;
        }
        string u;
        u+=s[i];
        u+=s[i+1];
        if(i==n-2){
            if(k==0 && s.back()!='0'){
                ans.push_back(s[i]-'1'+'a');
                ans.push_back(s[i+1]-'1'+'a');
                break;
            }else{
                ans.push_back(stoi(u)-1+'a');
                break;
            }
        }
        if(stoi(u)>26)ans.push_back(s[i]-'1'+'a');
        else{
            if(k>=p[i+1]){
                k-=p[i+1];
                ans.push_back(stoi(u)-1+'a');
                i++;
            }else{
                ans.push_back(s[i]-'1'+'a');
            }
        }
    }
    cout << ans << endl;

    return 0;
}