#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef pair<int, int> P;

template <int m> ostream& operator<<(ostream& os, const static_modint<m>& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const dynamic_modint<m>& a) {os << a.val(); return os;}
template <int m> istream& operator>>(istream& is, static_modint<m>& a) {long long x; is >> x; a = x; return is;}
template <int m> istream& operator>>(istream& is, dynamic_modint<m>& a) {long long x; is >> x; a = x; return is;}
template<typename T> istream& operator>>(istream& is, vector<T>& v){int n = v.size(); assert(n > 0); rep(i, n) is >> v[i]; return is;}
template<typename U, typename T> ostream& operator<<(ostream& os, const pair<U, T>& p){os << p.first << ' ' << p.second; return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : " "); return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : ""); return os;}
template<typename T> ostream& operator<<(ostream& os, const set<T>& se){for(T x : se) os << x << " "; os << "\n"; return os;}
template<typename T> ostream& operator<<(ostream& os, const unordered_set<T>& se){for(T x : se) os << x << " "; os << "\n"; return os;}
template<typename S, auto op, auto e> ostream& operator<<(ostream& os, const atcoder::segtree<S, op, e>& seg){int n = seg.max_right(0, [](S){return true;}); rep(i, n) os << seg.get(i) << (i == n - 1 ? "\n" : " "); return os;}
template<typename S, auto op, auto e, typename F, auto mapping, auto composition, auto id> ostream& operator<<(ostream& os, const atcoder::lazy_segtree<S, op, e, F, mapping, composition, id>& seg){int n = seg.max_right(0, [](S){return true;}); rep(i, n) os << seg.get(i) << (i == n - 1 ? "\n" : " "); return os;}

template<typename T> void chmin(T& a, T b){a = min(a, b);}
template<typename T> void chmax(T& a, T b){a = max(a, b);}

const long long INF = 2002002002002002002;

int main(){
	int n;
	cin >> n;
	long long k;
	cin >> k;
	string s;
	cin >> s;
	
	vector<long long> dp(n + 1);
	dp[n] = 1;
	for(int i = n - 1; i >= 0; i--){
		if(s[i] == '0'){
			dp[i] = 0;
			continue;
		}
		if(i < n - 1 and (s[i] - '0') * 10 + (s[i + 1] - '0')<= 26){
			dp[i] = dp[i + 1] + dp[i + 2];
		}else{
			dp[i] = dp[i + 1];
		}
		if(dp[i] > INF) dp[i] = INF;
	}
	
	string ans;
	int i = 0;
	while(i < n){
		if(s[i] == '0') assert(false);
		if(i < n - 1 and (s[i] - '0') * 10 + (s[i + 1] - '0') <= 26){
			if(k <= dp[i + 1]){
				char c = (s[i] - '0') - 1 + 'a';
				ans += c;
				i++;
			}else{
				char c = (s[i] - '0') * 10 + (s[i + 1] - '0') - 1 + 'a';
				ans += c;
				k -= dp[i + 1];
				i += 2;
			}
		}else{
			char c = (s[i] - '0') - 1 + 'a';
			ans += c;
			i++;
		}
	}
	cout << ans;
	cerr << dp;
	
	
	return 0;
}