#include <iostream>
using namespace std;
 
const int MAX_LENGTH  = 1000;
const int MAX_VAL = 100000;
int freq[MAX_VAL + MAX_VAL + 1];
 
 
int main() {
	int n,val;
	cin >> n;
	for (int i = 1; i <= n; ++i) {
		cin >> val;
		int index = val + MAX_VAL;
		if (freq[index] == 1) {
			cout << val;
			return 0;
		}
		freq[index] = 1;

	}
	cout << "NU";
	return 0;
}