#include <bits/stdc++.h>
using namespace std;
int p[200] = {0};
int main() {
  string a;
  int m;
  cin >> m;
  cin >> a;
  int n = a.length();
  for (int i = 0; i < n; i++) {
    if (a[i] != '?') p[a[i] - 'a'] = 1;
    if (a[i] != '?' && a[i] - 'a' >= m) {
      cout << "IMPOSSIBLE" << endl;
      return 0;
    }
  }
  for (int i = 0; i < n; i++) {
    if (a[i] == '?' && a[n - i - 1] != '?') a[i] = a[n - i - 1];
  }
  for (int i = n / 2; i >= 0; i--) {
    if (a[i] == '?' && a[n - i - 1] == '?') {
      for (int j = m - 1; j >= 0; j--) {
        if (!p[j]) {
          a[i] = j + 'a';
          a[n - i - 1] = j + 'a';
          p[j] = 1;
          break;
        }
      }
      if (a[i] == '?') a[i] = 'a', a[n - i - 1] = 'a';
    }
  }
  for (int i = 0; i < n; i++) {
    if (a[i] != a[n - i - 1]) {
      cout << "IMPOSSIBLE" << endl;
      return 0;
    }
  }
  for (int i = 0; i < m; i++)
    if (!p[i]) {
      cout << "IMPOSSIBLE" << endl;
      return 0;
    }
  cout << a << endl;
  return 0;
}
