#include <bits/stdc++.h>
using namespace std;
char alphz[27] = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
                  'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r',
                  's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
float Euclidean(long long x1, long long x2, long long y1, long long y2) {
  return sqrt(pow(x1 - x2, 2) + pow(y1 - y2, 2));
}
long long gcd(long long a, long long b) { return !b ? a : gcd(b, a % b); }
long long LCM(long long a, long long b) { return (a * b) / gcd(a, b); }
void PrimeFactor(long long n) {
  while (n % 2 == 0) {
    n /= 2;
  }
  for (int i = 3; i <= sqrt(n); i += 2) {
    if (n % i == 0) {
      n /= i;
    }
  }
  if (n > 2) {
  };
}
bool is_square(long long x) {
  long long l = 0, r = x;
  while (l <= r) {
    long long mid = l + (r - l) / 2;
    if (mid * mid == x) return true;
    if (mid * mid > x)
      r = mid - 1;
    else
      l = mid + 1;
  }
  return false;
}
long long power(long long x, long long y) {
  long long temp;
  if (y == 0) return 1;
  temp = power(x, y / 2);
  if (y % 2 == 0)
    return temp * temp;
  else
    return x * temp * temp;
}
bool is_Prime(int x) {
  if (x == 2)
    return 1;
  else if (x % 2 == 0 || x < 2)
    return 0;
  for (int i = 3; i * i <= x; i += 2)
    if (x % i == 0) return 0;
  return 1;
}
const int N = 1e+5;
int aseg[4], seg[4];
void SEGBuild(int pos, int low, int hig) {
  if (low == hig)
    seg[pos] = aseg[low];
  else {
    int mid = low + (hig - low) / 2;
    SEGBuild(2 * pos, low, mid);
    SEGBuild(2 * pos + 1, mid + 1, hig);
    if (seg[2 * pos] < seg[2 * pos + 1])
      seg[pos] = seg[2 * pos];
    else
      seg[pos] = seg[2 * pos + 1];
  }
}
void SEGUpdate(int pos, int idx, int low, int hig, int val) {
  if (low == hig) {
    aseg[idx] = val;
    seg[pos] = val;
  } else {
    int mid = low + (hig - low) / 2;
    if (idx >= low && idx <= mid)
      SEGUpdate(2 * pos, idx, low, mid, val);
    else
      SEGUpdate(2 * pos + 1, idx, mid + 1, hig, val);
    seg[pos] = min(seg[2 * pos + 1], seg[2 * pos]);
  }
}
int ANSQuery(int pos, int low, int hig, int l, int r) {
  if (l > hig || low > r) return 1e+5;
  if (l <= low && r >= hig)
    return seg[pos];
  else {
    int p1, p2;
    int mid = low + (hig - low) / 2;
    p1 = ANSQuery(2 * pos, low, mid, l, r);
    p2 = ANSQuery(2 * pos + 1, mid + 1, hig, l, r);
    return min(p1, p2);
  }
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int k;
  string s;
  cin >> k >> s;
  vector<int> a(k, 0);
  int len = (int)s.size();
  if (len == 1) {
    if (k > 1) return cout << "IMPOSSIBLE\n", 0;
    return cout << "a" << '\n', 0;
  }
  int i = 0, j = len - 1;
  while (i < j) {
    if (s[i] != '?' && s[j] != '?' && s[i] != s[j])
      return cout << "IMPOSSIBLE\n", 0;
    if (s[i] != '?') a[s[i] - 'a']++;
    if (s[j] != '?') a[s[j] - 'a']++;
    i++, j--;
  }
  int mid = len / 2;
  if (len % 2 == 0)
    i = mid - 1, j = mid;
  else
    i = mid, j = mid;
  while (i >= 0) {
    if (s[i] != '?' && s[j] != '?')
      a[s[i] - 'a'] += 2;
    else if (s[i] == '?' && s[j] != '?')
      s[i] = s[j], a[s[j] - 'a'] += 2;
    else if (s[i] != '?' && s[j] == '?')
      s[j] = s[i], a[s[i] - 'a'] += 2;
    else {
      int idx = 0;
      for (int i = k - 1; i >= 0; i--) {
        if (a[i] == 0) {
          idx = i;
          break;
        }
      }
      s[i] = s[j] = idx + 'a';
      a[idx] += 2;
    }
    i--, j++;
  }
  for (int i = 0; i < k; i++)
    if (!a[i]) return cout << "IMPOSSIBLE\n", 0;
  cout << s << '\n';
  return 0;
}
