void solve() {
int k = nextInt();
char[] c = nextToken().toCharArray();
for (int i = 0, j = c.length - 1; i < j; i++, j--) {
if (c[i] != '?' && c[j] != '?' && c[i] != c[j]) {
out.println("IMPOSSIBLE");
return;
}
}
int q = 0;
TreeSet<Character> ts = new TreeSet<Character>();
for (int i = 0; i < k; i++) {
ts.add((char) ('a' + i));
}
for (int i = 0, j = c.length - 1; i <= j; i++, j--) {
if (c[i] == '?' && c[j] == '?') {
q++;
}
if (c[i] != '?') {
ts.remove(c[i]);
}
if (c[j] != '?') {
ts.remove(c[j]);
}
}
for (int i = (c.length - 1) / 2, j = c.length - i - 1; i >= 0; i--, j++) {
if (c[i] == '?' && c[j] == '?') {
if (!ts.isEmpty()) {
c[i] = c[j] = ts.pollLast();
} else {
c[i] = c[j] = 'a';
}
}
}
if (!ts.isEmpty()) {
out.println("IMPOSSIBLE");
return;
}
for (int i = 0, j = c.length - 1; i < j; i++, j--) {
if (c[i] == '?') {
c[i] = c[j];
} else if (c[j] == '?') {
c[j] = c[i];
}
}
out.println(new String(c));
}
