#include <bits/stdc++.h>
using namespace std;
int main() {
  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = (int)s.size();
  int c = 0;
  vector<int> m(26);
  for (int i = 0; i < n / 2 + (n % 2 == 0 ? 0 : 1); i++) {
    if (s[i] != s[n - i - 1]) {
      if (s[i] != '?' && s[n - i - 1] != '?') {
        cout << "IMPOSSIBLE\n";
        return 0;
      } else if (s[i] == '?') {
        s[i] = s[n - i - 1];
      } else {
        s[n - i - 1] = s[i];
      }
      m[s[i] - 'a'] = 1;
    } else {
      if (s[i] != '?') {
        m[s[i] - 'a'] = 1;
      } else {
        c++;
      }
    }
  }
  vector<int> v;
  for (int i = 0; i < k; i++) {
    if (m[i] == 0) {
      v.push_back(i);
    }
  }
  reverse(v.begin(), v.end());
  if (c < (int)v.size()) {
    cout << "IMPOSSIBLE";
    return 0;
  }
  for (int i = 0; i < n; i++) {
    if (s[i] == '?') {
      if (c > (int)v.size()) {
        s[i] = 'a';
        s[n - i - 1] = 'a';
      } else {
        s[i] = (char)('a' + v.back());
        s[n - i - 1] = (char)('a' + v.back());
        v.pop_back();
      }
      c--;
    }
  }
  cout << s;
}
