#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iostream>
#include <cctype>
#include <sstream>
#include <string>
#include <list>
#include <vector>
#include <queue>
#include <set>
#include <stack>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <iterator>
#include <bitset>
#include <complex>
#include <fstream>
#include <iomanip>
using namespace std;
typedef long long ll;
const double EPS = 1e-9;
typedef vector<int> vint;
typedef pair<int, int> pint;
#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for(int i = x; i < n; i++)
template<class T> T RoundOff(T a){ return int(a+.5-(a<0)); }
template<class T, class C> void chmax(T& a, C b){ if(a < b) a = b; }
template<class T, class C> void chmin(T& a, C b){ if(b < a) a = b; }
template<class T, class C> pair<T, C> mp(T a, C b){ return make_pair(a, b); }

int main()
{
	int n;
	while(cin >> n && n)
	{
		vint a(26, 0);
		vector<vint> G(26, vint(26));

		rep(i, n)
		{
			string word;
			cin >> word;

			a[word[0] - 'a']++;
			a[word[word.size() - 1] - 'a']++;
			G[word[0] - 'a'][word[word.size() - 1] - 'a']++;
		}


		// AOtÅ é©Ç¤©
		int s;
		vint used = a;
		queue<int> que;
		rep(i, 26)
			s = a[i] ? i : s;
		used[s] = 0;
		for(que.push(s); !que.empty(); que.pop())
		{
			int p = que.front();

			rep(i, 26)
			{
				if(G[p][i] && used[i])
				{
					used[i] = 0;
					que.push(i);
				}
			}
		}

		if(0 < accumulate(ALL(used), 0))
		{
			cout << "NG" << endl;
			goto ng;
		}


		// êMÅ«é©Ç¤©
		rep(i, 26)
		{
			if(a[i] % 2 == 1)
			{
				cout << "NG" << endl;
				goto ng;
			}
		}

		cout << "OK" << endl;
ng:
		;
	}
}