#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <iomanip>
#include <numeric>
using namespace std;
using ll = long long;
char toc(int a) {
	char now = 'a';
	a--;
	now += a;
	return now;
}
int main() {
	int N;ll K;cin >> N >> K;
	string T;cin >> T;
	vector<ll> dp(N+3, 0);
	dp[N] = 1;
	dp[N+1] = 1;
	for (int i = N-1;i >= 0;i--) {
		//split by one pattern
		int a = T[i] - '0';
		if (a != 0) dp[i] = min(K+2, dp[i+1]);
		//split by two pattern
		if (i+1 < N) {
			int b = T[i+1] - '0';
			if (a != 0 and ((10*a)+b) <= 26) {
				dp[i] = min(K+2, dp[i] + dp[i+2]);
			}
		}
	}
	string ret = "";
	int it = 0;
	while (it < N) {
		int a = T[it] - '0';
		if (a != 0) {
			ll dp1 = dp[it+1];
			if (dp1 >= K) {
				ret += toc(a);
				it++;
				continue;
			}
			K -= dp1;
		}
		int b = T[it+1] - '0';
		int c = (10 * a) + b;
		ret += toc(c);
		it += 2;
	}
	cout << ret << endl;
}