/**
	author:  shobonvip
	created: 2025.10.03 21:20:02
**/

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

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)
#define all(v) v.begin(), v.end()

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	ll n, k;
	cin >> n >> k;
	k--;
	vector<int> a(n);
	rep(i,0,n) {
		char x; cin >> x;
		a[i] = x - '0';
	}
	
	vector<ll> dp(n + 1);
	dp[n] = 1;
	rrep(i,0,n) {
		if (i + 1 < n) {
			if (
				1 <= a[i] && a[i] <= 9
				&& 1 <= a[i] * 10 + a[i+1]
				&& a[i] * 10 + a[i+1] <= 26
			) {
				dp[i] += dp[i+2];
			}
		}
		if (1 <= a[i] && a[i] <= 9) {
			dp[i] += dp[i+1];
		}
		chmin(dp[i], ll(2e18));
	}

	int i = 0;
	vector<char> ans;
	while (i < n) {
		if (1 <= a[i] && a[i] <= 9) {
			if (dp[i+1] <= k) {
				k -= dp[i+1];
			} else {
				ans.push_back(char(a[i] - 1 + 'a'));
				i = i + 1;
				continue;
			}
		}

		if (i + 1 < n) {
			if (
				1 <= a[i] && a[i] <= 9
				&& 1 <= a[i] * 10 + a[i+1]
				&& a[i] * 10 + a[i+1] <= 26
			) {
				ans.push_back(char(a[i] * 10 + a[i+1] - 1 + 'a'));
				i = i + 2;
				continue;
			}
		}
	}

	for (char c: ans) cout << c;
	cout << '\n';

}