#include <fstream>
#include <algorithm>

using namespace std;

ifstream f( "date.in" );
ofstream g( "date.out" );

int v[10001], n;

void p1(){
    int x, y, v[10001], v2[10001], k = 0;
    f >> x;
    f >> y;
    v[1] = x;
    v2[1] = v[1];
    for( int i = 2; v[i-1] < y; ++i ){
        if( i % 2 == 0 ) v[i] = v[i-1] + 1;
        else v[i] = v[i-2] * 2 + 1;
    }
    for( int i = 1; v[i-1] < y; ++i ){
        v2[++k] = v[i];
    }
    for( int i = k; i >= 1; --i ){
        g << v2[i] << " ";
    }
    g << '\n';
}

void p2(){
    int x, y, v[10001], v2[10001], k = 1;
    f >> x;
    v[1] = 1;
    v2[1] = v[1];
    for( int i = 2; v[i-1] < x; ++i ){
        if( i % 2 == 0 ) v[i] = v[i-1] + 1;
        else v[i] = v[i-2] * 2 + 1;
        v2[++k] = v[i];
    }
    sort( v2 + 1, v2 + k + 1 );
    for( int i = k; i >= 1; --i ){
        g << v2[i] << " ";
    }
    g << '\n';
}

int sir( int k ){
    if( k <= n ){
        v[k] = 1 - 2 * v[k-1] - v[k-2];
        sir( k + 1 );
        g << v[k] << " ";
    }
}

void p3(){
    f >> n;
    v[1] = 1;
    v[2] = -1;
    if( n == 1 ) g << 1 << '\n';
    else sir( 3 ), g << -1 << " " << 1 << '\n';
}

void p4(){
    int n, k = 0, x, gasit, v[10001], dif;
    f >> n;
    for( int i = 1; i <= n; ++i ){
        f >> x;
        gasit = 0;
        for( int j = 1; j <= k && !gasit; ++j ){
            if( x == v[j] ) gasit = 1;
        }
        if( !gasit ) v[++k] = x;
    }
    sort( v + 1, v + k + 1 );
    dif = v[2] - v[1];
    gasit = 1;
    for( int i = 2; i <= k && gasit; ++i ){
        if( v[i] - v[i-1] != dif ) gasit = 0;
    }
    if( gasit ) g << dif << '\n';
    else g << "NU" << '\n';
}

int main()
{
    p4();
}