#include <cstdio>
#include <cstdlib>
int n,res = -1;
char str[2000005];
void upload(int x){
if(~res){
puts("NOT UNIQUE");
exit(0);
}
res = x;
}
void output(){
if(res == -1) puts("NOT POSSIBLE");
else{
int p = 0;
for (int i=0; i<n/2+p; i++) {
if(i == res){
p++;
continue;
}
putchar(str[i]);
}
}
}
void calc_m(){
for (int i=0; i<n/2; i++) {
if(str[i] != str[i+n/2+1]) return;
}
res = n/2;
}
char s1[1000004], s2[1000004], diff[1000004], cnt;
void calc_l(){
cnt = 0;
for (int i=0; i<n/2; i++) {
s1[i] = str[i];
s2[i] = str[i+n/2+1];
diff[i] = (s1[i] != s2[i]);
cnt += diff[i];
}
for (int i=n/2-1; i>=0; i--) {
s1[i] = str[i+1];
if(s1[i] == s2[i] && diff[i]){
diff[i] = 0;
cnt--;
}
if(s1[i] != s2[i] && !diff[i]){
diff[i] = 1;
cnt++;
}
if(cnt == 0) upload(i);
}
}
void calc_r(){
cnt = 0;
for (int i=0; i<n/2; i++) {
s1[i] = str[i];
s2[i] = str[i+n/2+1];
diff[i] = (s1[i] != s2[i]);
cnt += diff[i];
}
for (int i=n/2+1; i<n; i++) {
s2[i-n/2-1] = str[i-1];
if(s1[i-n/2-1] == s2[i-n/2-1] && diff[i-n/2-1]){
diff[i-n/2-1] = 0;
cnt--;
}
if(s1[i-n/2-1] != s2[i-n/2-1] && !diff[i-n/2-1]){
diff[i-n/2-1] = 1;
cnt++;
}
if(cnt == 0) upload(i);
}
}
int main(){
scanf("%d %s",&n,str);
if(!(n&1)){
puts("NOT POSSIBLE");
exit(0);
}
calc_m();
calc_l();
calc_r();
output();
}
