#include <iostream>
#include <cstring>
using namespace std;
const int maxn=1000*2;
char s[maxn],t[maxn];
int main(){
    int n;
    while(cin>>n){
        cin>>s;
        memcpy(t,s,sizeof(s));
        int low,high,cnt=0;
        for(low=0,high=n-1;low<high;low++,high--)
            cnt+=(s[low]==s[high]&&s[low]!='?'?1:0);
        if(cnt==n/2){
            cout<<"QwQ"<<endl;
            continue;
        }
        for(int i=0;i<n;i++)
            t[i]=(s[i]=='?'?'a':s[i]);
        for(cnt=0,low=0,high=n-1;low<high;low++,high--)
            cnt+=(t[low]==t[high]?1:0);
        if(cnt==n/2){
            for(int i=n-1;i>=0;i--)
                if(s[i]=='?'){
                    if(n%2==1&&n/2==i)
                        continue;
                    t[i]=(t[n-i-1]=='a'?'b':'a');
                    break;
                }
        }

        cout<<t<<endl;
    }
    return 0;
}