#include <bits/stdc++.h>
using namespace std;
int main() {
  int k;
  string s;
  cin >> k >> s;
  int l = s.size();
  vector<int> q;
  set<char> u;
  for (int i = 0; i < k; i++) u.insert(i + 'a');
  for (char c : s)
    if (c != '?') u.erase(c);
  for (int i = 0; i < l / 2; i++)
    if (s[i] == '?' && s[l - i - 1] == '?') {
      q.push_back(i);
    } else if (s[i] == '?') {
      s[i] = s[l - i - 1];
    } else if (s[l - i - 1] == '?') {
      s[l - i - 1] = s[i];
    } else if (s[i] != s[l - i - 1]) {
      cout << "IMPOSSIBLE\n";
      return 0;
    }
  if (l % 2 != 0 && s[l / 2] == '?') q.push_back(l / 2);
  if (q.size() < u.size()) {
    cout << "IMPOSSIBLE\n";
    return 0;
  }
  reverse(q.begin(), q.end());
  for (int i : q) {
    if (!u.empty()) {
      s[i] = s[l - i - 1] = *u.rbegin();
      u.erase(--u.end());
    } else {
      s[i] = s[l - i - 1] = 'a';
    }
  }
  cout << s << '\n';
}
