#include <bits/stdc++.h>

#include <atcoder/fenwicktree.hpp>
#include <atcoder/segtree.hpp>
#include <atcoder/modint.hpp>
#include <atcoder/dsu.hpp>
#include <atcoder/lazysegtree.hpp>

using namespace atcoder;
using namespace std;
using ll = long long;
using ull = unsigned long long;
template <class T>
using max_heap = priority_queue<T>;
template <class T>
using min_heap = priority_queue<T, vector<T>, greater<>>;
ll ll_min = numeric_limits<ll>::min();
ll ll_max = numeric_limits<ll>::max();
ll ALPHABET_N = 26;
using mint = modint998244353;
#define rep(i, n) for (ll i = (ll)0; i < (ll)n; i++)
#define rep_(i, k, n) for (ll i = (ll)k; i < (ll)n; i++)
#define all(a) a.begin(), a.end()

char strnum_to_char(string str)
{
	return (char)((stoi(str) - 1) + 'a');
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	ll n, k;
	cin >> n >> k;
	string t;
	cin >> t;
	if (n == 1)
	{
		cout << strnum_to_char(t) << endl;
		return 0;
	}
	if (n == 2)
	{
		string ans = "";
		if (k == 1)
		{
			ans += strnum_to_char(t.substr(0, 1));
			ans += strnum_to_char(t.substr(1, 1));
		}
		else
			ans += strnum_to_char(t.substr(0, 2));
		cout << ans << endl;
		return 0;
	}
	vector<vector<ll>> patterns(n, vector<ll>(2, 0));
	for (ll i = n - 1; i >= 0; i--)
	{
		if (i == n - 1)
		{
			if (t[i] != '0')
				patterns[i][0] = 1;
			continue;
		}
		char c = t[i];
		if (c != '0')
		{
			patterns[i][0] = patterns[i + 1][0] + patterns[i + 1][1];
		}
		if (c == '1' || (c == '2' && t[i + 1] <= '6'))
		{
			if (i + 2 < n)
				patterns[i][1] = patterns[i + 2][0] + patterns[i + 2][1];
			else
				patterns[i][1] = 1;
		}
		patterns[i][0] = min(patterns[i][0], ll_max / 10);
		patterns[i][1] = min(patterns[i][1], ll_max / 10);
	}
	string ans = "";
	ll rest = k;
	rest--;
	for (ll i = 0; i < n;)
	{
		if (rest >= patterns[i][0])
		{
			rest -= patterns[i][0];
			ans += strnum_to_char(t.substr(i, 2));
			i += 2;
		}
		else
		{
			ans += strnum_to_char(t.substr(i, 1));
			i += 1;
		}
	}
	cout << ans << endl;
	return 0;
}