#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n; cin >> n;
	long long k; cin >> k;
	string t; cin >> t;
	vector<long long>dp(n + 1);
	dp[n] = 1;
	vector<char>c1(n), c2(n + 1);
	rrep(i, n) {
		if (t[i] == '0')continue;
		if (i != n - 1) {
			int x = t[i] - '0';
			x *= 10;
			x += t[i + 1] - '0';
			if (0 < x && x <= 26) {
				dp[i] += dp[i + 2];
				c2[i] = 'a' + x - 1;
			}
		}
		{
			int x = t[i] - '0';
			if (0 < x && x <= 26) {
				dp[i] += dp[i + 1];
				c1[i] = 'a' + x - 1;
			}
		}
		chmin(dp[i], LINF * 2);
	}
	string ans;
	for (int i = 0; i < n;) {
		if (dp[i + 1] >= k) {
			ans += c1[i];
			i++;
		}
		else {
			k -= dp[i + 1];
			ans += c2[i];
			i += 2;
		}
	}
	cout << ans << endl;
	return 0;
}