//#define _GLIBCXX_DEBUG

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

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (ll)(m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9+7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T1, typename T2>
bool chmin(T1 &a,T2 b){if(a>b){a=b;return true;}else return false;}
template<typename T1, typename T2>
bool chmax(T1 &a,T2 b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,-1,0,0,1,1,-1,-1};
vector<ll>dy={0,0,1,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2>&p){
  return os << p.first << " " << p.second;
}
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());

int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0,buf=0;
  bool judge = true;
  ll n;cin>>n;
  string s;cin>>s;
  vector<ll>a,b,c;
  rep(i,0,2*n){
    if(s[i]=='a'){
      c.PB(a.size());
      a.PB(i);
    }
    else{
      c.PB(b.size());
      b.PB(i);
    }
  }
  auto f=[&](string t){
    ll nxt=0;
    queue<ll>ch;
    stack<ll>st;
    rep(i,0,2*n){
      if(nxt==t.size())break;
      if(!ch.empty()&&ch.front()==i){
        ch.pop();
        if(s[i]==t[nxt]){
          st.push(c[i]);
          nxt++;
        }
        else{
          ll mi=min(a[c[i]],b[c[i]]);
          while(!st.empty()){
            ll p=st.top();
            if(max(a[p],b[p])>mi){
              chmin(mi,min(a[p],b[p]));
              nxt-=2;
              st.pop();
            }
            else break;
          }
          nxt--;
        }
      }
      else if(min(a[c[i]],b[c[i]])==i&&s[i]==t[nxt]){
        nxt++;
        ch.push(max(a[c[i]],b[c[i]]));
      }
    }
    //cout<<t spa nxt<<endl;
    return nxt==t.size();
  };
  string ret;
  while(1){
    if(f(ret+'b')){
      ret+='b';
    }
    else if(f(ret+'a')){
      ret+='a';
    }
    else break;
  }
  cout<<ret<<endl;
  return 0;
}

Measured memory_bytes: 512