#include<bits/stdc++.h>
using namespace std;

int main(){
  using ll=long long;
  ll inf=2e18;
  ll n,k;
  string t;
  cin>>n>>k>>t;
  auto f=[&](int l,int len){
    if (!(0<=l&&l+len<=n)) return -1;
    int ret;
    if (len==1) ret=t[l]-'0';
    if (len==2){
      if (t[l]=='0') ret=-1;
      else ret=(t[l]-'0')*10+(t[l+1]-'0');
    }
    if (1<=ret&&ret<=26) return ret;
    else return -1;
  };
  vector<ll> dp(n+1);
  dp[0]=1;
  for (int i=1;i<=n;i++){
    if (f(n-i,1)!=-1) dp[i]+=dp[i-1];
    if (f(n-i,2)!=-1) dp[i]+=dp[i-2];
    dp[i]=min(dp[i],inf);
  }
  string ans;
  for (int i=0;i<n;){
    vector<tuple<int,ll,int>> vp;
    if (f(i,1)!=-1) vp.push_back({f(i,1),dp[n-i-1],i+1});
    if (f(i,2)!=-1) vp.push_back({f(i,2),dp[n-i-2],i+2});
    sort(vp.begin(),vp.end());
    ll sum=0;
    for (auto [val,s,j]:vp){
      if (k<=sum+s){
        ans+=char('a'+val-1);
        i=j;
        k-=sum;
        break;
      }
      sum+=s;
    }
  }
  cout<<ans<<endl;
}