#include<algorithm>
#include<cassert>
#include<climits>
#include<cmath>
#include<cstring>
#include<deque>
#include<functional>
#include<iomanip>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<vector>

using namespace std;

using lint = long long;
using P = pair<int, int>;
using LLP = pair<long long, long long>;

#define REP(i, x, n) for(int i = (x), i##_len = int(n) ; i < i##_len ; ++i)
#define rep(i, n) for(int i = 0, i##_len = int(n) ; i < i##_len ; ++i)
#define reps(i, n) for(int i = 1, i##_len = int(n) ; i <= i##_len ; ++i)
#define rrep(i, n) for(int i = int(n) - 1 ; i >= 0 ; --i)
#define rreps(i, n) for(int i = int(n) ; i > 0 ; --i)
#define SORT(x) sort((x).begin(), (x).end())
#define SORT_INV(x) sort((x).rbegin(), (x).rend())
#define TWINS(x) cout << ((x) ? "Yay!" : ":(") << endl

constexpr int IINF = (1 << 30) - 1;
constexpr long long LLINF = 1LL << 61;
constexpr double EPS = 1e-8;

const int dx4[] = {1, 0, -1, 0}, dy4[] = {0, 1, 0, -1};
const int dx8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dy8[] = {0, -1, -1, -1, 0, 1, 1, 1};


class UnionFind{
    vector<int> par;
    vector<long long> wei;
    int groupCount;

public:
    UnionFind(int n = 0){
        init(n);
    }

    void init(int n = 0){
        par.resize(n);
        fill(par.begin(), par.end(), -1);

        wei.resize(n);
        fill(wei.begin(), wei.end(), 0LL);

        groupCount = n;

        return;
    }

    int root(int x){
        if(par[x] < 0){
            return x;
        }
        int r = root(par[x]);
        wei[x] += wei[par[x]];
        return par[x] = r;
    }

    bool same(int x, int y){
        return root(x) == root(y);
    }

    int size(int x){
        return -par[root(x)];
    }

    long long weight(int x){
        root(x);
        return wei[x];
    }

    long long diff(int x, int y){
        return weight(y) - weight(x);
    }

    bool unite(int x, int y, long long w = 0){
        w += weight(x) - weight(y);

        x = root(x);
        y = root(y);

        if(x == y){
            return false;
        }

        if(par[y] < par[x]){
            swap(x, y);
            w = -w;
        }

        par[x] += par[y];
        par[y] = x;
        wei[y] = w;
        --groupCount;

        return true;
    }

    int size(void){
        return groupCount;
    }
};

template<typename T>
bool chmax(T &a, T b){
    if(a < b){
        a = b;
        return true;
    }
    return false;
}

template<typename T>
bool chmin(T &a, T b){
    if(b < a){
        a = b;
        return true;
    }
    return false;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n;
    while(cin >> n, n){
        vector<int> f(26), b(26);
        UnionFind uf(26);
        set<char> st;
        rep(i, n){
            string s;
            cin >> s;
            ++f[s.front() - 'a'];
            ++b[s.back() - 'a'];
            uf.unite(s.front() - 'a', s.back() - 'a');
            st.insert(s.front());
            st.insert(s.back());
        }

        bool flag = uf.size() == (26 - (int)st.size() + 1);
        rep(i, 26){
            if(f[i] != b[i]){
                flag = false;
                break;
            }
        }

        if(flag){
            cout << "OK" << endl;
        }else{
            cout << "NG" << endl;
        }
    }

    return 0;
}


Measured memory_bytes: 3124