This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include<iostream>
#include<string.h>
using namespace std;
const int MAX_N=2000001;
int n;
char str[MAX_N+1];
int main(){
	cin >> n >> str;
	int i,tmp=0;
	for(i=0; i<n/2 && str[i]==str[n/2+i]; i++);
	for(; i<n/2 && str[i]==str[n/2+1+i]; i++);
	if(i==n/2) tmp+=1;
	for(i=0; i<n/2 && str[i]==str[n/2+1+i]; i++);
	for(; i<n/2 && str[i+1]==str[n/2+1+i]; i++);
	if(i==n/2) tmp+=2;
	str[n/2]=0;
	if(tmp==0 || n%2==0) cout<<"NOT POSSIBLE";
	else if(tmp==3 && strcmp(str,str+n/2+1)) cout << "NOT UNIQUE";
	else if(tmp==1) cout<<str;
	else cout<<str+n/2+1;
	return 0;
}
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