解法：

暴力

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
#define endl '\n'
bool cmp(string& a, string& b) {
	if (a.size() != b.size())
		return a.size() > b.size();
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	int n; cin >> n;
	vector<string> s(n);
	for (int i = 0; i < n; i++) cin >> s[i];
	string t0 = s[0];
	sort(s.begin(), s.end(), cmp);
	for (int i = 1; i < n; i += 2) {
		if (s[i-1].size() != s[i].size()) {
			cout << "NO" << endl;
			return 0;
		}
	}
	cout << t0 << endl;
	return 0;
}