#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-8;
const int dx[] = {1, 0, -1, 0, -1, -1, 1, 1};
const int dy[] = {0, 1, 0, -1, -1, 1, -1, 1};
const int OO = INT_MAX;
int main() {
  int k;
  cin >> k;
  string s;
  cin >> s;
  int mid = ((int)s.size() + 1) / 2;
  int n = (int)s.size() - 1;
  int cnt = 0;
  for (int i = 0; i < mid; i++) {
    if (s[i] == '?' && s[n - i] != '?') {
      s[i] = s[n - i];
    } else if (s[i] != '?' && s[n - i] == '?') {
      s[n - i] = s[i];
    } else if (s[i] != '?' && s[n - i] != '?') {
      if (s[i] != s[n - i]) {
        cout << "IMPOSSIBLE" << '\n';
        return 0;
      }
    } else {
      cnt++;
    }
  }
  vector<char> notApperedYet;
  for (int c = 0; c < k; c++) {
    char ch = c + 'a';
    if (s.find(ch) == -1) {
      notApperedYet.push_back(ch);
    }
  }
  if ((int)notApperedYet.size() > cnt) {
    cout << "IMPOSSIBLE" << '\n';
    return 0;
  }
  while (cnt > (int)notApperedYet.size()) {
    for (int i = 0; i < mid; i++) {
      if (s[i] == '?') {
        s[i] = s[n - i] = 'a';
        break;
      }
    }
    cnt--;
  }
  int index = 0;
  for (int i = 0; i < mid; i++) {
    if (s[i] == '?' && s[n - i] == '?') {
      s[i] = s[n - i] = notApperedYet[index++];
    }
  }
  cout << s << '\n';
  return 0;
}
