#if __INCLUDE_LEVEL__ == 0

#include __BASE_FILE__

void Solve() {
  int n;
  int64_t K;
  IN(n, K);
  string s;
  IN(s);

  vector<int64_t> cnt(n + 1);
  cnt[n] = 1;
  for (int i : Rev(Rep(0, n))) {
    if (s[i] != '0') {
      cnt[i] = cnt[i + 1];
    }
    if (i + 2 <= n &&
        (s[i] == '1' ||
         (s[i] == '2' && s[i + 1] <= '6'))) {
      cnt[i] += cnt[i + 2];
    }
    SetMin(cnt[i], INF64);
  }

  string ans;
  ans.reserve(n);

  int i = 0;
  while (i < n) {
    assert(K <= cnt[i]);
    if (s[i] != '0') {
      if (K <= cnt[i + 1]) {
        ans += char('a' + (s[i] - '1'));
        ++i;
        continue;
      } else {
        K -= cnt[i + 1];
      }
    }

    if (i + 2 <= n &&
        (s[i] == '1' ||
         (s[i] == '2' && s[i + 1] <= '6'))) {
      assert(K <= cnt[i + 2]);
      ans += char('a' + stoi(s.substr(i, 2)) - 1);
      i += 2;
    }
  }

  OUT(ans);
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  Solve();
}

#elif __INCLUDE_LEVEL__ == 1

#include <bits/stdc++.h>

template <class T> concept MyRange = std::ranges::range<T> && !std::convertible_to<T, std::string_view>;
template <class T> concept MyTuple = std::__is_tuple_like<T>::value && !MyRange<T>;

namespace std {

istream& operator>>(istream& is, MyRange auto&& r) {
  for (auto&& e : r) is >> e;
  return is;
}
istream& operator>>(istream& is, MyTuple auto&& t) {
  apply([&](auto&... xs) { (is >> ... >> xs); }, t);
  return is;
}

ostream& operator<<(ostream& os, MyRange auto&& r) {
  auto sep = "";
  for (auto&& e : r) os << exchange(sep, " ") << e;
  return os;
}
ostream& operator<<(ostream& os, MyTuple auto&& t) {
  auto sep = "";
  apply([&](auto&... xs) { ((os << exchange(sep, " ") << xs), ...); }, t);
  return os;
}

}  // namespace std

using namespace std;

#define LAMBDA2(x, y, ...) ([&](auto&& x, auto&& y) -> decltype(auto) { return __VA_ARGS__; })
#define Rev views::reverse
#define Rep(...) [](int l, int r) { return views::iota(min(l, r), r); }(__VA_ARGS__)
#define SetMin(...) LAMBDA2(x, y, y < x && (x = y, 1))(__VA_ARGS__)
#define INF64 (INT64_MAX / 2)
#define IN(...) (cin >> forward_as_tuple(__VA_ARGS__))
#define OUT(...) (cout << forward_as_tuple(__VA_ARGS__) << '\n')

#endif  // __INCLUDE_LEVEL__ == 1