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

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    
    long long N,K; cin >> N >> K;
    string s; cin >> s;
    vector<long long> dp(N+1);
    dp.at(N) = 1;
    long long inf = 2e18;
    if(s.back() != '0') dp.at(N-1) = 1;
    for(int i=N-2; i>=0; i--){
        int v = (s.at(i)-'0')*10+(s.at(i+1)-'0');
        if(1 <= v && v <= 26) dp.at(i) += dp.at(i+2);
        v /= 10;
        if(1 <= v && v < 10) dp.at(i) += dp.at(i+1);
        dp.at(i) = min(inf,dp.at(i));
        if(s.at(i) == '0') dp.at(i) = 0;
    }
    string answer = ""; K--;
    for(int i=0; i<N; i++){
        if(dp.at(i+1) <= K){
            K -= dp.at(i+1);
            int v = (s.at(i)-'0')*10+(s.at(i+1)-'0');
            answer += v-1+'a'; i++;
        }
        else answer += s.at(i)-'1'+'a';
    }
    cout << answer << endl;
}