// #define _GLIBCXX_DEBUG // for STL debug (optional)
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <string>
#include <cstring>
#include <deque>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <complex>
#include <cmath>
#include <limits>
#include <cfloat>
#include <climits>
#include <ctime>
#include <cassert>
#include <numeric>
#include <fstream>
#include <functional>
#include <bitset>
using namespace std;
using ll = long long int;
using int64 = long long int;
 
template<typename T> void chmax(T &a, T b) {a = max(a, b);}
template<typename T> void chmin(T &a, T b) {a = min(a, b);}
template<typename T> void chadd(T &a, T b) {a = a + b;}
 
int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};
const int INF = 1LL << 29;
const ll LONGINF = 1LL << 60;
const ll MOD = 1000000007LL;
 
ll N, K;
ll dp[300010];

int main() {
    cin >> N >> K;
    string T; cin >> T;
    fill(dp, dp + N + 1, 0);
    dp[N] = 1;
    for(int i=N-1; i>=0; i--) {
        for(int c=1; c<=2; c++) {
            if (i + c > N) continue;
            int num = stoi(T.substr(i, c));
            if (num <= 0 || num > 26 || (int)to_string(num).size() != c) continue;
            dp[i] = min(K+1, dp[i] + dp[i+c]);
        }
    }

    int pos = 0;
    ll rem = K;
    while (pos < N) {
        // fprintf(stderr, "pos = %d, rem = %lld\n", pos, rem);
        assert(dp[pos] >= rem);
        for(int c=1; c<=2; c++) {
            if (pos + c > N) continue;
            int num = stoi(T.substr(pos, c));
            if (num <= 0 || num > 26 || (int)to_string(num).size() != c) continue;
            if (rem > dp[pos + c]) {
                rem -= dp[pos + c];
            } else {
                cout << (char)('a' + num - 1);
                pos += c;
                break;
            }
        }
    }
    assert(rem == 1);
    cout << endl;
    return 0;
}