/* -*- coding: utf-8 -*-
 *
 * 3290.cc:  No.3290 Encrypt Failed, but Decrypt Succeeded - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 300000;

/* typedef */

using ll = long long;

/* global variables */

char t[MAX_N + 4], s[MAX_N + 4];
ll dp[MAX_N + 1];

/* subroutines */

inline int d1(char t[], int i) { return t[i] - '0'; }
inline int d2(char t[], int i) { return d1(t, i) * 10 + d1(t, i + 1); }

/* main */

int main() {
  int n;
  ll k;
  scanf("%d%lld%s", &n, &k, t);
  k--;

  dp[n] = 1;
  for (int i = n - 1; i >= 0; i--) {
    if (t[i] == '0')
      dp[i] = 0;
    else {
      dp[i] = dp[i + 1];
      if (i + 2 <= n && d2(t, i) <= 26) dp[i] += dp[i + 2];
    }
  }

  int m = 0;
  for (int i = 0; i < n;) {
    if (k < dp[i + 1]) {
      s[m++] = 'a' + d1(t, i) - 1;
      i++;
    }
    else {
      s[m++] = 'a' + d2(t, i) - 1;
      k -= dp[i + 1];
      i += 2;
    }
  }
  s[m] = '\0';

  puts(s);
  
  return 0;
}