#include <bits/stdc++.h>
using namespace std;
int k;
string s;
int main() {
  cin >> k;
  cin >> s;
  int n = s.length();
  for (int i = 0; i < n; ++i) {
    if (s[i] == '?' && s[n - i - 1] == '?') {
      continue;
    } else if (s[i] == '?') {
      s[i] = s[n - i - 1];
    } else if (s[n - i - 1] == '?') {
      s[n - i - 1] = s[i];
    } else if (s[i] != s[n - i - 1]) {
      cout << "IMPOSSIBLE";
      return 0;
    }
  }
  bool flag[26];
  memset(flag, 0, sizeof(flag));
  for (int i = 0; i < n; ++i) {
    if (s[i] != '?') {
      flag[s[i] - 'a'] = true;
    }
  }
  int miss = 0;
  for (int c = 0; c < k; ++c) {
    if (!flag[c]) {
      ++miss;
    }
  }
  int have = 0;
  for (int i = 0; i <= n - i - 1; ++i) {
    if (s[i] == '?') {
      ++have;
    }
  }
  if (miss > have) {
    cout << "IMPOSSIBLE";
    return 0;
  }
  for (int i = 0; i < n; ++i) {
    if (s[i] == '?') {
      if (have > miss) {
        s[i] = s[n - i - 1] = 'a';
      } else {
        for (int c = 0; c < k; ++c) {
          if (!flag[c]) {
            s[i] = s[n - i - 1] = c + 'a';
            flag[c] = true;
            break;
          }
        }
        --miss;
      }
      --have;
    }
  }
  cout << s << endl;
  return 0;
}
