#include <bits/stdc++.h>
using namespace std;
int main() {
  int k;
  cin >> k;
  string s;
  cin >> s;
  set<int> has;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '?') {
      s[i] = s[s.size() - i - 1];
    } else {
      if (s[s.size() - i - 1] != '?' && s[s.size() - i - 1] != s[i]) {
        cout << "IMPOSSIBLE" << endl;
        return 0;
      }
      has.insert(s[i] - 'a');
    }
  }
  int j = k - 1;
  while (has.count(j)) {
    j--;
  }
  for (int i = s.size() / 2; i >= 0; i--) {
    if (s[i] == '?') {
      if (j >= 0) {
        s[i] = s[s.size() - i - 1] = 'a' + j;
        j--;
        while (has.count(j)) {
          j--;
        }
      } else {
        s[i] = s[s.size() - i - 1] = 'a';
      }
    }
  }
  if (j >= 0) {
    cout << "IMPOSSIBLE" << endl;
  } else {
    cout << s;
  }
  return 0;
}
