#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define vi vector<int>
#define vl vector<ll>
#define vii vector<pair<int,int>>
#define vll vector<pair<ll,ll>>
#define vvi vector<vector<int>>
#define vvl vector<vector<ll>>
#define vvii vector<vector<pair<int,int>>>
#define vvll vector<vector<pair<ll,ll>>>
#define vst vector<string>
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define mkunique(x) sort(all(x));(x).erase(unique(all(x)),(x).end())
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=300005;
const ll INF=15LL<<57;

ll dp[MAX];

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    ll N,K;cin>>N>>K;
    string S;cin>>S;
    dp[N]=1;
    for(int i=N-1;i>=0;i--){
        {
            ll x=(ll)(S[i]-'0');
            if(1<=x&&x<=26) dp[i]+=dp[i+1];
        }
        if(i<=N-2){
            ll x=(ll)(S[i]-'0')*10+(ll)(S[i+1]-'0');
            if(S[i]!='0'&&1<=x&&x<=26) dp[i]+=dp[i+2];
        }
        chmin(dp[i],INF);
    }
    
    string ans;
    
    int i=0;
    while(i<N){
        {
            ll x=(ll)(S[i]-'0');
            if(1<=x&&x<=26){
                if(K<=dp[i+1]){
                    ans+=char('a'+(x-1));
                    i++;
                    continue;
                }else{
                    K-=dp[i+1];
                }
            }
        }
        {
            ll x=(ll)(S[i]-'0')*10+(ll)(S[i+1]-'0');
            if(S[i]!='0'&&1<=x&&x<=26){
                if(K<=dp[i+2]){
                    ans+=char('a'+(x-1));
                    i+=2;
                    continue;
                }
            }
        }
    }
    cout<<ans<<endl;
}