/*{{{*/
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<string>
#include<iostream>
#include<sstream>
#include<set>
#include<map>
#include<queue>
#include<bitset>
#include<vector>
#include<limits.h>
#include<assert.h>
#define SZ(X) ((int)(X).size())
#define ALL(X) (X).begin(), (X).end()
#define REP(I, N) for (int I = 0; I < (N); ++I)
#define REPP(I, A, B) for (int I = (A); I < (B); ++I)
#define FOR(I, A, B) for (int I = (A); I <= (B); ++I)
#define FORS(I, S) for (int I = 0; S[I]; ++I)
#define RS(X) scanf("%s", (X))
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define GET_POS(c,x) (lower_bound(c.begin(),c.end(),x)-c.begin())
#define CASET int ___T; scanf("%d", &___T); for(int cs=1;cs<=___T;cs++)
#define MP make_pair
#define PB push_back
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define LEN(X) strlen(X)
#define F first
#define S second
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<PII> VPII;
typedef pair<LL,LL> PLL;
typedef vector<PLL> VPLL;
template<class T> void _R(T &x) { cin >> x; }
void _R(int &x) { scanf("%d", &x); }
void _R(LL &x) { scanf("%lld", &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }
void R() {}
template<class T, class... U> void R(T &head, U &... tail) { _R(head); R(tail...); }
template<class T> void _W(const T &x) { cout << x; }
void _W(const int &x) { printf("%d", x); }
void _W(const LL &x) { printf("%lld", x); }
void _W(const double &x) { printf("%.16f", x); }
void _W(const char &x) { putchar(x); }
void _W(const char *x) { printf("%s", x); }
template<class T,class U> void _W(const pair<T,U> &x) {_W(x.F); putchar(' '); _W(x.S);}
template<class T> void _W(const vector<T> &x) { for (auto i = x.begin(); i != x.end(); _W(*i++)) if (i != x.cbegin()) putchar(' '); }
void W() {}
template<class T, class... U> void W(const T &head, const U &... tail) { _W(head); putchar(sizeof...(tail) ? ' ' : '\n'); W(tail...); }
#ifdef HOME
 #define DEBUG(...) {printf("# ");printf(__VA_ARGS__);puts("");}
#else
 #define DEBUG(...)
#endif
int MOD = 1e9+7;
void ADD(LL& x,LL v){x=(x+v)%MOD;if(x<0)x+=MOD;}
/*}}}*/
const int SIZE = 6e3+10;
char s[SIZE];
int far[SIZE];
PII pp[SIZE];
string dp[SIZE];
int id[SIZE];
char tmp[SIZE];
int main(){
    int n; R(n);
    RS(s);
    int it[2]={};
    REP(i,2*n){
        if(s[i]=='a'){
            id[i]=it[0];
            pp[it[0]++].F=i;
        }
        else {
            id[i]=it[1];
            pp[it[1]++].S=i;
        }
    }
    for(int i=n-1;i>=0;i--){
        if(pp[i].F<pp[i].S){
            dp[i]="ab";
            REPP(j,i+1,n){
                if(min(pp[j].F,pp[j].S)>pp[i].S){
                    dp[i]="ab"+dp[j];
                    break;
                }
            }
        }
        else{
            if(i+1<n&&pp[i+1].S<pp[i].F&&far[i+1])far[i]=far[i+1];
            else far[i]=i;
            int len=0;
            REPP(j,pp[i].S,2*n){
                if(id[j]>=i&&id[j]<=far[i])tmp[len++]=s[j];
            }
            tmp[len]=0;
            dp[i]=string(tmp);
            REPP(j,far[i]+1,n){
                if(min(pp[j].F,pp[j].S)>pp[far[i]].F){
                    dp[i]+=dp[j];
                    break;
                }
            }
        }
        if(i+1<n)dp[i]=max(dp[i],dp[i+1]);
    }
    W(dp[0]);
    return 0;
}


Measured memory_bytes: 10624