#include <bits/stdc++.h>

using namespace std;

int main()
{
  int n;
  cin>>n;
  string s;
  cin>>s;
    unordered_set<char> s1;
    
  for(int i=n-1;i>=0;i--)
  {
    if(s1.find(s[i])==s1.end()) {
        s1.insert(s[i]);
    }
    else if(s[i]==max(s[i],s[i+1])){
        s1.insert(s[i]);
    }
    
  }
  string ans;
  for(auto it=s1.begin();it!=s1.end();it++){
      ans+=*it;
  }
  cout<<ans<<endl;
    return 0;
}