#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

int N;
ll K;
string S;
ll pat[303030];
void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>K>>S;
	
	pat[N]=1;
	for(i=N-1;i>=0;i--) {
		if(S[i]=='0') {
			pat[i]=0;
		}
		else {
			pat[i]=pat[i+1];
			if(S[i]=='1'||(S[i]=='2'&&S[i+1]<='6')) pat[i]+=pat[i+2];
			pat[i]=min(pat[i],1LL<<61);
		}
	}
	
	int cur=0;
	while(cur<N) {
		if(K<=pat[cur+1]) {
			cout<<(char)('a'+S[cur]-'1');
			cur++;
		}
		else {
			K-=pat[cur+1];
			cout<<(char)('a'+(S[cur]-'0')*10+S[cur+1]-'1');
			cur+=2;
			
		}
	}
	cout<<endl;
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}