Vasya has recently finished writing a book. Now he faces the problem of giving it the title. Vasya wants the title to be vague and mysterious for his book to be noticeable among others. That's why the title should be represented by a single word containing at least once each of the first k Latin letters and not containing any other ones. Also, the title should be a palindrome, that is it should be read similarly from the left to the right and from the right to the left.

Vasya has already composed the approximate variant of the title. You are given the title template s consisting of lowercase Latin letters and question marks. Your task is to replace all the question marks by lowercase Latin letters so that the resulting word satisfies the requirements, described above. Each question mark should be replaced by exactly one letter, it is not allowed to delete characters or add new ones to the template. If there are several suitable titles, choose the first in the alphabetical order, for Vasya's book to appear as early as possible in all the catalogues.

Input

The first line contains an integer k (1 ≤ k ≤ 26) which is the number of allowed alphabet letters. The second line contains s which is the given template. In s only the first k lowercase letters of Latin alphabet and question marks can be present, the length of s is from 1 to 100 characters inclusively.

Output

If there is no solution, print IMPOSSIBLE. Otherwise, a single line should contain the required title, satisfying the given template. The title should be a palindrome and it can only contain the first k letters of the Latin alphabet. At that, each of those k letters must be present at least once. If there are several suitable titles, print the lexicographically minimal one. 

The lexicographical comparison is performed by the standard < operator in modern programming languages. The line a is lexicographically smaller than the line b, if exists such an i (1 ≤ i ≤ |s|), that ai < bi, and for any j (1 ≤ j < i) aj = bj. |s| stands for the length of the given template.

Examples

Input

3
a?c


Output

IMPOSSIBLE


Input

2
a??a


Output

abba


Input

2
?b?a


Output

abba
import static java.lang.Math.*;
import java.util.*;
import java.io.*;

public class _55_d2_C {

    boolean showDebug = true;

    public void solve() throws Exception {

        int k = nextInt();
        char[] s = nextWord().toCharArray();

        boolean[] ok = new boolean[150];
        int need = k;
        for (char c='a'; c<'a'+k; c++) {
            if (occurenciesCnt(new String(s), c)>0) {
                ok[c]=true;
                need--;
            }
        }
        int free = 0;
        for (int i=0; i<(s.length/2+(odd(s.length)?1:0)); i++) {
            if (s[i]=='?' && s[s.length-1-i]=='?') free++;
        }

        if (free<need) {println("IMPOSSIBLE"); return;}

        for (int i=0; i<s.length; i++) {
            if (s[i]=='?') {
                if (s[s.length-1-i]=='?') {
                    char c = 'a';
                    if (free>need) {
                        if (!ok['a']) {
                            ok['a']=true;
                            need--;
                        }
                        free--;
                    } else {
                        for (char cc='a'; cc<'a'+k; cc++) {
                            if (!ok[cc]) {
                                c=cc;
                                ok[cc]=true;
                                break;
                            }
                        }
                    }
                    s[i]=c;
                    s[s.length-1-i]=c;
                } else {
                    s[i]=s[s.length-1-i];
                }
            }
        }
        //      debug(s);
        boolean okk = true;
        for (char c='a'; c<'a'+k; c++) if (!ok[c]) okk=false;
        for (char c: s) if (c>'a'+k) okk=false;
        if (!isPalindrome(new String(s)) || !okk) {println("IMPOSSIBLE"); return;}
        else println(new String(s));

    }



    ////////////////////////////////////////////////////////////////////////////    

    double EPS = 1e-7;
    int INF = Integer.MAX_VALUE;
    long INFL = Long.MAX_VALUE;
    double INFD = Double.MAX_VALUE;

    int[] dx = {0,1,0,-1};
    int[] dy = {-1,0,1,0};
    int[] dx8 = {0,1,1,1,0,-1,-1,-1};
    int[] dy8 = {-1,-1,0,1,1,1,0,-1};
    int[] knightMovesX = {1,2,2,1,-1,-2,-2,-1};
    int[] knightMovesY = {-2,-1,1,2,2,1,-1,-2};


    @SuppressWarnings("serial")
    class IncMap extends HashMap<Object, Integer> {
        boolean add(Object key, int amount) {
            Integer i = get(key);
            if (i!=null) {
                put(key, i+amount);
                return false;
            } else {
                put(key, amount);
                return true;
            }
        }
        boolean add(Object key) {
            return add(key, 1);
        }
    }


    int min(int... nums) {
        int r = INF;
        for (int i: nums)
            if (i<r) r=i;
        return r;
    }
    int max(int... nums) {
        int r = -INF;
        for (int i: nums)
            if (i>r) r=i;
        return r;
    }
    long minL(long... nums) {
        long r = INFL;
        for (long i: nums)
            if (i<r) r=i;
        return r;
    }
    long maxL(long... nums) {
        long r = -INFL;
        for (long i: nums)
            if (i>r) r=i;
        return r;
    }
    double minD(double... nums) {
        double r = INFD;
        for (double i: nums)
            if (i<r) r=i;
        return r;
    }
    double maxD(double... nums) {
        double r = -INFD;
        for (double i: nums)
            if (i>r) r=i;
        return r;
    }
    long sumArr(int[] arr) {
        long res = 0;
        for (int i: arr)
            res+=i;
        return res;
    }
    long sumArr(long[] arr) {
        long res = 0;
        for (long i: arr)
            res+=i;
        return res;
    }
    double sumArr(double[] arr) {
        double res = 0;
        for (double i: arr)
            res+=i;
        return res;
    }


    long partsFitCnt(long partSize, long wholeSize) {
        return (partSize+wholeSize-1)/partSize;
    }

    boolean odd(long i) {
        return (i&1)==1;
    }

    int digitSum(long i) {
        i = abs(i);
        int r = 0;
        while (i>0) {
            r+=i%10;
            i/=10;
        }
        return r;
    }
    long digitProd(long i) {
        if (i==0) return 0;
        i = abs(i);
        long r = 1;
        while (i>0) {
            r*=i%10;
            i/=10;
        }
        return r;
    }

    long gcd (long a, long b) {
        while (b>0) {
            a%=b;
            a^=b; b^=a; a^=b;
        }
        return a;
    }
    long lcm(long a, long b) {
        return (a*b)/gcd(a,b);
    }

    double log_2 = log(2);
    double log2(double i) {
        return log(i)/log_2;
    }
    long binpow(int x, int n) {
        long r = 1;
        while (n>0) {
            if ((n&1)!=0) r*=x;
            x*=x;
            n>>=1;
        }
        return r;
    }
    long fac(int i) {
        if (i>20) throw new IllegalArgumentException();
        return i<=1 ?  1:fac(i-1)*i;
    }


    double dist(double x, double y, double xx, double yy) {
        return sqrt((xx-x)*(xx-x)+(yy-y)*(yy-y));
    }


    boolean isPalindrome(String s) {
        for (int i=0; i<s.length()/2; i++)
            if (s.charAt(i)!=s.charAt(s.length()-1-i)) return false;
        return true;
    }
    int occurenciesCnt(String s, String pattern) {
        int res = 0;
        for (int i=0; i<s.length()-pattern.length()+1; i++)
            if (s.substring(i, i+pattern.length()).equals(pattern)) res++;
        return res;
    }
    int occurenciesCnt(String s, char pattern) {
        int res = 0;
        for (int i=0; i<s.length(); i++)
            if (s.charAt(i)==pattern) res++;
        return res;
    }


    int[] months = {0,31,28,31,30,31,30,31,31,30,31,30,31};
    boolean isLeapYear(int y) {
        return y%4==0 && (y%400==0 || y%100!=0);
    }
    boolean isValidDate(int d, int m, int y) {
        if (isLeapYear(y) && m==2 && d==29) return true;
        return m>0 && m<=12 && d>0 && d<=months[d];
    }
    int[] nextDay(int d, int m, int y) {
        if (d>=months[m])
            if (m==2 && d==28 && isLeapYear(y)) d++;
            else {d=1; m++;}
        else
            d++;
        if (m==13) {d=1; m=1; y++;}
        return new int[] {d,m,y};
    }

    String str(Object o) {
        return o.toString();
    }


    long timer = System.currentTimeMillis();
    void startTimer() {
        timer = System.currentTimeMillis();
    }
    void stopTimer() {
        System.err.println("time: "+(System.currentTimeMillis()-timer)/1000.0);
    }


    static BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    static BufferedWriter out = new BufferedWriter(new OutputStreamWriter(System.out));

    String nextLine() throws IOException {
        return in.readLine();
    }
    String nextWord() throws IOException {
        StringBuilder sb = new StringBuilder();
        int c = 0;
        while (c<=' ') c=in.read();
        while (c>' ') {
            sb.append((char)c);
            c = in.read();
        }
        return sb.toString();
    }
    int nextInt() throws NumberFormatException, IOException {
        return Integer.parseInt(nextWord());
    }
    long nextLong() throws NumberFormatException, IOException {
        return Long.parseLong(nextWord());
    }
    double nextDouble() throws NumberFormatException, IOException {
        return Double.parseDouble(nextWord());
    }
    int[] nextArr(int size) throws NumberFormatException, IOException {
        int[] arr = new int[size];
        for (int i=0; i<size; i++)
            arr[i] = nextInt();
        return arr;
    }
    long[] nextArrL(int size) throws NumberFormatException, IOException {
        long[] arr = new long[size];
        for (int i=0; i<size; i++)
            arr[i] = nextLong();
        return arr;
    }
    double[] nextArrD(int size) throws NumberFormatException, IOException {
        double[] arr = new double[size];
        for (int i=0; i<size; i++)
            arr[i] = nextDouble();
        return arr;
    }
    String[] nextArrS(int size) throws NumberFormatException, IOException {
        String[] arr = new String[size];
        for (int i=0; i<size; i++)
            arr[i] = nextWord();
        return arr;
    }

    void print(Object o) throws IOException {
        out.write(o.toString());
    }
    void println(Object o) throws IOException {
        out.write(o.toString());
        out.newLine();
    }
    void print(Object... o) throws IOException {
        for (int i=0; i<o.length; i++) {
            if (i!=0) out.write(' ');
            out.write(o[i].toString());
        }
    }
    void println(Object... o) throws IOException {
        print(o);
        out.newLine();
    }
    void printn(Object o, int n) throws IOException {
        String s = o.toString();
        for (int i=0; i<n; i++) {
            out.write(s);
            if (i!=n-1) out.write(' ');
        }
    }
    void printnln(Object o, int n) throws IOException {
        printn(o, n);
        out.newLine();
    }
    void printArr(int[] arr) throws IOException {
        for (int i=0; i<arr.length; i++) {
            if (i!=0) out.write(' ');
            out.write(Integer.toString(arr[i]));
        }
    }
    void printArr(long[] arr) throws IOException {
        for (int i=0; i<arr.length; i++) {
            if (i!=0) out.write(' ');
            out.write(Long.toString(arr[i]));
        }
    }
    void printArr(double[] arr) throws IOException {
        for (int i=0; i<arr.length; i++) {
            if (i!=0) out.write(' ');
            out.write(Double.toString(arr[i]));
        }
    }
    void printArr(String[] arr) throws IOException {
        for (int i=0; i<arr.length; i++) {
            if (i!=0) out.write(' ');
            out.write(arr[i]);
        }
    }
    void printlnArr(int[] arr) throws IOException {
        printArr(arr);
        out.newLine();
    }
    void printlnArr(long[] arr) throws IOException {
        printArr(arr);
        out.newLine();
    }
    void printlnArr(double[] arr) throws IOException {
        printArr(arr);
        out.newLine();
    }
    void printlnArr(String[] arr) throws IOException {
        printArr(arr);
        out.newLine();
    }

    void debug(Object... o) {
        if (showDebug) System.err.println(Arrays.deepToString(o));
    }


    public static void main(String[] args) throws Exception {
        Locale.setDefault(Locale.US);
        new _55_d2_C().solve();
        out.flush();
        out.close(); in.close();
    }
}
