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 java.io.*;
import java.math.BigInteger;
import java.util.*;


 public class Main {
	public static InputReader in;
    public static PrintWriter pw;



	public static void main(String args[]) {
		new Thread(null, new Runnable() {
            public void run() {
                try{
                    solve();
                }
                catch(Exception e){
                    e.printStackTrace();
                }
            }
        }, "1", 1 << 26).start();
    }
	static TreeSet<Integer> set;
	static ArrayList<Pair> g[];
	static boolean visited[];
	static long edje=0;
    static boolean vis[];
	static int parent[];
	static int col[];
	static boolean[] vis1;
	static int Ans=0;
	static int min=Integer.MAX_VALUE;
	static int val[];
	static int degree[];
	static int a[];
	static int f[];
	static  long total=0;
	static HashSet<String> s;
	static int distance[];
	public static void solve(){
	
		 in = new InputReader(System.in);
		 pw = new PrintWriter(System.out);
	     int k=in.nextInt();
	     String s=in.readString();
	     char a[]=s.toCharArray();
	     for(int i=0;i<(s.length()/2);i++)
	     {
	    	 if(a[i]!=a[s.length()-1-i]&&a[i]!='?'&&a[s.length()-1-i]!='?')
	    	 {
	    		 System.out.println("IMPOSSIBLE");
	    		 System.exit(0);
	    	 }
	     }
	     for(int i=0;i<s.length();i++)
	     {
	    	 if(a[i]!='?')
	    	 {
	    		 if((a[i]-'a')>k)
	    		 {
	    			 System.out.println("IMPOSSIBLE");
		    		 System.exit(0);
	    		 }
	    	 }
	     }
	     int Que=0;
	     for(int i=0;i<(s.length());i++)
	     {
	    	 if(a[i]=='?')
	    		 Que++;
	     }
	     if(Que==0)
	     {
	    	 StringBuilder sb=new StringBuilder(s);
	    	 if(sb.reverse().toString().equals(s))
	    	 {
	    		 int p=0;
	    		 boolean bool[]=new boolean[26];
	    		 for(int i=0;i<s.length();i++)
	    		 {
	    			 if((a[i]-'a')<k&&!bool[a[i]-'a']){
	    				 p++;bool[a[i]-'a']=true;
	    			 }
	    		 }
	    		 if(p==k)
	    		 {
	    			 System.out.println(s);
	    		 }
	    		 else
	    			 System.out.println("IMPOSSIBLE");
	    		 System.exit(0);
	    	 }
	    	 else
	    		 System.out.println("IMPOSSIBLE");
	    	 System.exit(0);
	     }
	     if(s.length()%2==0)
	     {
	     boolean ar[]=new boolean[26];	 
	     int Used=0;
	     for(int i=0;i<s.length();i++)
	     {
	    	   if(a[i]=='?'&&a[s.length()-1-i]!='?')
	    	   {
	    		   a[i]=a[s.length()-1-i];
	    		   
	    		   if(!ar[a[s.length()-1-i]-'a'])
	    		   {
	    		   ar[a[s.length()-1-i]-'a']=true;
	    		   Used++;
	    		   }
	    	   }
	    	   else if(a[s.length()-1-i]=='?'&&a[i]!='?')
	    	   {
	    		   
	    		   a[s.length()-1-i]=a[i];
	    		   if(!ar[a[i]-'a'])
	    		   {
	    		   ar[a[i]-'a']=true;
	    		   Used++;
	    		   }
	    	   }
	    	   else if(a[i]!='?')
	    	   {
	    		   ar[a[i]-'a']=true;
	    	   }
	     }
	     TreeSet<Character> st=new TreeSet<Character>();
	     for(char c='a';c<('a'+k);c++)
	     {
	    	 if(!ar[c-'a'])
	    		 st.add(c);
	     }
	     for(int i=((s.length()/2)-1);i>=0;i--)
	     {
	    	 if(a[i]=='?'&&a[s.length()-1-i]=='?')
	    	 {
	    		 a[i]=st.last();
	    		 a[s.length()-1-i]=st.last();
	    		 st.remove(st.last());
	    	 }
	    	 if(st.size()==0)
	    		 break;
	     }
	     for(int i=0;i<s.length();i++)
	     {
	    	 if(a[i]=='?'&&a[s.length()-1-i]=='?')
	    	 {
	    		 a[i]='a';
	    		 a[s.length()-1-i]='a';
	    	 }
	     }
	     HashSet<Character> set=new HashSet<Character>();
	     for(int i=0;i<s.length();i++)
	     {
	    	 if((a[i]-'a')<k)
	    		 set.add(a[i]);
	     }
	     if(set.size()==k)
	     System.out.println(new String(a));
	     else
	    	 System.out.println("IMPOSSIBLE");
	     System.exit(0);
	     }
	     else
	     {
	    	// System.out.println("ODD");
	    	 boolean ar[]=new boolean[26];	 
		     int Used=0;
		     for(int i=0;i<(s.length()/2);i++)
		     {
		    	   if(a[i]=='?'&&a[s.length()-1-i]!='?')
		    	   {
		    		   a[i]=a[s.length()-1-i];
		    		   
		    		   if(!ar[a[s.length()-1-i]-'a'])
		    		   {
		    		   ar[a[s.length()-1-i]-'a']=true;
		    		   Used++;
		    		   }
		    	   }
		    	   else if(a[s.length()-1-i]=='?'&&a[i]!='?')
		    	   {
		    		   
		    		   a[s.length()-1-i]=a[i];
		    		   if(!ar[a[i]-'a'])
		    		   {
		    		   ar[a[i]-'a']=true;
		    		   Used++;
		    		   }
		    	   }
		    	   else if(a[i]!='?')
		    	   {
		    		   ar[a[i]-'a']=true;
		    	   }
		     }
		     TreeSet<Character> st=new TreeSet<Character>();
		     for(char c='a';c<('a'+k);c++)
		     {
		    	 if(!ar[c-'a'])
		    		 st.add(c);
		     }
		     if(a[s.length()/2]=='?'&&st.size()>0)
		     {
		    	 a[s.length()/2]=st.last();
		    	 st.remove(st.last());
		     }
		     for(int i=((s.length()/2)-1);i>=0;i--)
		     {
		    	 if(a[i]=='?'&&a[s.length()-1-i]=='?'&&st.size()>0)
		    	 {
		    		 a[i]=st.last();
		    		 a[s.length()-1-i]=st.last();
		    		 st.remove(st.last());
		    	 }
		    	 if(st.size()==0)
		    		 break;
		     }
		     
		     for(int i=0;i<s.length();i++)
		     {
		    	 if(a[i]=='?'&&a[s.length()-1-i]=='?')
		    	 {
		    		 a[i]='a';
		    		 a[s.length()-1-i]='a';
		    	 }
		     }
		     HashSet<Character> set=new HashSet<Character>();
		     for(int i=0;i<s.length();i++)
		     {
		    	 if((a[i]-'a')<k)
		    		 set.add(a[i]);
		     }
		     if(set.size()==k)
		     System.out.println(new String(a));
		     else
		    	 System.out.println("IMPOSSIBLE");
		    
		     System.exit(0);

	     }
    }

	/*public static void dfs(int curr)
	{
		vis[curr]=true;
		int trees=0;
		for(int x:g[curr])
		{
			if(!vis[x]&&x!=1)
			{
			trees++;
			if(!s.contains(curr+" "+x))
				total+=cost[curr][x];
			dfs(x);
			}
	    }
		if(trees==0)
		{
			if(!s.contains(curr+" "+1))
				total+=cost[1][curr];
		}
	}
	public static void dfs1(int curr,int parent)
	{
	val[curr]=a[curr];
	for(int x:g[curr])
	{
	       if(x!=parent)
	       {
	    	   dfs1(x,curr);
	    	   val[curr]+=val[x];
	      }
	
	}
	}
	public static void dfs2(int curr,int parent)
	{
		f[curr]=val[curr];
		for(int x:g[curr])
		{
			if(x!=parent)
			{
				dfs2(x,curr);
				f[curr]+=f[x];
			}
		}
	}
	
	
	 
/*    public static void dfs2(int curr)
    {
    if(st[col[curr]].isEmpty())
    {
    ans[curr]=-1;	
    }
    else
    {
    	ans[curr]=st[col[curr]].peek();
    }
    st[col[curr]].push(curr);
    for(int x:g[curr])
    {
    	dfs2(x);
    }
    st[col[curr]].pop();
    	
    }
    
  
	public static void dfs1(int curr)
	{
		visited[curr]=true;
		for(int next:g[curr])
		{
			edje++;
			if(!visited[next])
				dfs1(next);
		}
	}
	
	public static void dfs(int curr,int prev)
		{
		val[curr]=1;
			for(int x:g[curr])
			{
				if(x!=prev)
				{
		            dfs(x,curr);
		            val[curr]+=val[x];
					
			    }
			}
		}*/
	
		public static long power(long a,long b)
		{
			long result=1;
			while(b>0)
			{
				if(b%2==1)
					result*=a;
				a=a*a;
				b/=2;
			}
			return result;
		}
				public static long pow(long n,long p,long m)
	{
		 long  result = 1;
		  if(p==0)
		    return 1;
		if (p==1)
		    return n;
		while(p!=0)
		{
		    if(p%2==1)
		        result *= n;
		    if(result>=m)
		    result%=m;
		    p >>=1;
		    n*=n;
		    if(n>=m)
		    n%=m;
		}
		return result;
	}
	static class Pair implements Comparable<Pair>{
	int fir;
		int sec;
		Pair(int mr,int i){
	     	fir=mr;
			sec=i;
			
		}
		@Override
		public int compareTo(Pair o) {
		//	if(o.val<this.val)
		
			return 1;
			
			//else 
				//return -1;
			
			}
		
	}
	public static long gcd(long a, long b) {
		  if (b == 0) return a;
		  return gcd(b, a%b);
		}
	static class InputReader {
 
		private InputStream stream;
		private byte[] buf = new byte[8192];
		private int curChar, snumChars;
		private SpaceCharFilter filter;
 
		public InputReader(InputStream stream) {
			this.stream = stream;
		}
 
		public int snext() {
			if (snumChars == -1)
				throw new InputMismatchException();
			if (curChar >= snumChars) {
				curChar = 0;
				try {
					snumChars = stream.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (snumChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}
 
		public   int nextInt() {
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = snext();
			}
			int res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public long nextLong() {
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = snext();
			}
			long res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public int[] nextIntArray(int n) {
			int a[] = new int[n];
			for (int i = 0; i < n; i++)
				a[i] = nextInt();
			return a;
		}
 
		public String readString() {
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = snext();
			} while (!isSpaceChar(c));
			return res.toString();
		}
 
		public boolean isSpaceChar(int c) {
			if (filter != null)
				return filter.isSpaceChar(c);
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}
 
		public interface SpaceCharFilter {
			public boolean isSpaceChar(int ch);
		}
	}
}
