#include<bits/stdc++.h>
using namespace std;
#define N 2000005
long long bp1[N],hx1[N],p1=37,mod1=1e9+7,pre1,pre11,pre12,suf1,suf11,suf12,ans;
long long bp2[N],hx2[N],p2=31,mod2=3e9+7,pre2,pre21,pre22,suf2,suf21,suf22;
char s[N];
bool st,mid,en;
int main(){
int n,i;
scanf("%d %s",&n,s+1);
if(n%2==0){printf("NOT POSSIBLE");return 0;}
bp1[0]=bp2[0]=1;
for(i=1;i<=n;i++)bp1[i]=(bp1[i-1]*p1)%mod1;
for(i=1;i<=n;i++)bp2[i]=(bp2[i-1]*p2)%mod2;
for(i=1;i<=n;i++){
hx1[i]=(hx1[i-1]*p1+s[i]-'A'+1)%mod1;
hx2[i]=(hx2[i-1]*p2+s[i]-'A'+1)%mod2;
}
suf1=((hx1[n]-hx1[n/2+1]*bp1[n/2])%mod1+mod1)%mod1;
suf2=((hx2[n]-hx2[n/2+1]*bp2[n/2])%mod2+mod2)%mod2;
for(i=1;i<=n/2;i++){
pre11=(hx1[i-1]*bp1[n/2-i+1])%mod1;
pre12=(hx1[n/2+1]-hx1[i]*bp1[n/2+1-i])%mod1;
pre1=((pre11+pre12)%mod1+mod1)%mod1;
pre21=(hx2[i-1]*bp2[n/2-i+1])%mod2;
pre22=(hx2[n/2+1]-hx2[i]*bp2[n/2+1-i])%mod2;
pre2=((pre21+pre22)%mod2+mod2)%mod2;
if(pre1==suf1&&pre2==suf2)ans++,en=true;
}
pre1=hx1[n/2];
pre2=hx2[n/2];
if(pre1==suf1&&pre2==suf2)ans++,mid=true;
for(i=n/2+2;i<=n;i++){
suf11=((hx1[i-1]-hx1[n/2]*bp1[i-1-n/2])*bp1[n-i])%mod1;
suf12=(hx1[n]-hx1[i]*bp1[n-i])%mod1;
suf1=((suf11+suf12)%mod1+mod1)%mod1;
suf21=((hx2[i-1]-hx2[n/2]*bp2[i-1-n/2])*bp2[n-i])%mod2;
suf22=(hx2[n]-hx2[i]*bp2[n-i])%mod2;
suf2=((suf21+suf22)%mod2+mod2)%mod2;
if(pre1==suf1&&pre2==suf2)ans++,st=true;
}
if(ans>1)printf("NOT UNIQUE");
else if(ans){
if(st||mid)for(i=1;i<=n/2;i++)printf("%c",s[i]);
else for(i=n/2+2;i<=n;i++)printf("%c",s[i]);
}
else printf("NOT POSSIBLE");
return 0;
}
