#include <algorithm>
#include <cassert>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <iostream>
#include <numeric>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <functional>
#include <iomanip>
#include <ranges>
using namespace std;
using ll = long long;

template<typename T> auto range(T s, T e) { return views::iota(s, max(s, e)); }
template<typename T> auto range(T n) { return range<T>(0, n); }
template<typename T> void take(vector<T>& vec, int n) { vec.resize(n); for (int i = 0; i < n; ++i) cin >> vec[i]; }
template<class... Args> void sout(const Args &...args) { ((cout << args << ' '), ...); }
template<class... Args> void soutn(const Args &...args) { ((cout << args << ' '), ...); cout << '\n'; }
template<typename T1, typename T2> struct In2 {
  T1 a; T2 b; friend std::istream& operator>>(std::istream& is, In2& obj) { T1 t1; T2 t2; is >> t1 >> t2; obj = {t1, t2}; return is; } };
template<typename T1, typename T2, typename T3> struct In3 {
  T1 a; T2 b; T3 c; friend std::istream& operator>>(std::istream& is, In3& obj) { T1 t1; T2 t2; T3 t3; is >> t1 >> t2 >> t3; obj = {t1, t2, t3}; return is; } };
template<typename T1, typename T2, typename T3, typename T4> struct In4 {
  T1 a; T2 b; T3 c; T4 d; friend std::istream& operator>>(std::istream& is, In4& obj) { T1 t1; T2 t2; T3 t3; T4 t4; is >> t1 >> t2 >> t3 >> t4; obj = {t1, t2, t3, t4}; return is; } };

#ifdef LOCAL
#include <debug.h>
#else
#define dump(...) ;
#endif


namespace {

ll n, k; string t;

void read() {
  cin >> n >> k >> t;
}

int digit(char c1, char c2) {
  return (c1 - '0') * 10 + c2 - '0';
}

string run() {
  vector<ll> dp(n + 1);
  dp[n] = 1;
  for (int i = n - 1; i >= 0; --i) {
    if (t[i] != '0') {
      dp[i] += dp[i + 1];
      if (i < n - 1) {
        int d = digit(t[i], t[i + 1]);
        if (10 <= d && d <= 26) dp[i] += dp[i + 2];
      }
    }

    if (dp[i] > k) dp[i] = k + 1; // inf
  }
  dump(dp);

  string res;
  for (int i = 0; i < n; ) {
    assert(t[i] != '0');
    if (k <= dp[i + 1]) {
      res += 'a' + digit('0', t[i]) - 1;
      i++;
    } else {
      k -= dp[i + 1];
      res += 'a' + digit(t[i], t[i + 1]) - 1;
      i += 2;
    }
  }
  dump(res, k);
  assert(k == 1);
  return res;
}

}  // namespace

template <typename F>
void exec(F f) { if constexpr (std::is_same_v<decltype(f()), void>) f(); else cout << f() << endl; }

int main(int argc, char** argv) {
  cerr << fixed << setprecision(12);
  cout << fixed << setprecision(12);
  int testcase = 1;
  if (argc > 1) testcase = atoi(argv[1]);
  while (testcase--) {
    read();
  }
  exec(run);
}