#include <bits/stdc++.h>
using namespace std;
bool used[26];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int k;
  cin >> k;
  string s;
  cin >> s;
  int unset = 0;
  for (int i = 0; i < s.length() / 2; i++) {
    if (s[i] == '?') {
      if (s[s.length() - i - 1] != '?') {
        s[i] = s[s.length() - i - 1];
      } else {
        unset++;
      }
    } else if (s[s.length() - i - 1] == '?') {
      s[s.length() - i - 1] = s[i];
    } else if (s[i] != s[s.length() - i - 1]) {
      cout << "IMPOSSIBLE" << endl;
      return 0;
    }
  }
  if (s.length() % 2 == 1) {
    if (s[s.length() / 2] == '?') unset++;
  }
  int cnt = 0;
  for (int i = 0; i < s.length(); i++) {
    if (s[i] != '?') {
      if (!used[s[i] - 'a']) {
        used[s[i] - 'a'] = 1;
        cnt++;
      }
    }
  }
  if (unset < k - cnt) {
    cout << "IMPOSSIBLE" << endl;
    return 0;
  }
  int need = k - cnt;
  if (s.length() % 2 == 1 && s[s.length() / 2] == '?') {
    if (need > 0) {
      for (int j = k - 1; j >= 0; j--) {
        if (!used[j]) {
          s[s.length() / 2] = 'a' + j;
          need--;
          used[j] = 1;
          break;
        }
      }
    } else
      s[s.length() / 2] = 'a';
  }
  for (int i = s.length() / 2 - 1; i >= 0; i--) {
    if (s[i] == '?') {
      if (need > 0) {
        for (int j = k - 1; j >= 0; j--) {
          if (!used[j]) {
            s[i] = 'a' + j;
            need--;
            used[j] = 1;
            break;
          }
        }
      } else
        s[i] = 'a';
      s[s.length() - i - 1] = s[i];
    }
  }
  cout << s << endl;
  return 0;
}
