// @snippet<sh19910711/contest:headers.cpp>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
#include <numeric>
#include <limits>
#include <complex>
#include <functional>
#include <iterator>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>

// @snippet<sh19910711/contest:solution/interface.cpp>
namespace solution {
    class ISolution {
    public:
        virtual void init() {};
        virtual bool input() { return false; };
        virtual void output() {};
        virtual int run() = 0;
    };
}

// @snippet<sh19910711/contest:graph/config.cpp>
namespace graph {
    typedef int Weight;
    const Weight DEFAULT_WEIGHT = std::numeric_limits<Weight>().max() / 2;
}

// @snippet<sh19910711/contest:graph/interface.cpp>
namespace graph {
    class IGraph {
    public:
        virtual void init( int n ) = 0;
        virtual void addEdge( int from, int to ) = 0;
        virtual void addVertices() = 0;
        virtual void addEdges() = 0;
        virtual int countVertices() = 0;
        virtual Weight getEdgeCost( int from, int to ) = 0;
        virtual void setEdgeCost( int from, int to, Weight weight ) = 0;
        virtual Weight getVertexCost( int v ) = 0;
        virtual void setVertexCost( int v, Weight weight ) = 0;
    };
}

// @snippet<sh19910711/contest:graph/adjacent_list.cpp>
namespace graph {
    class AdjacentListGraph: public IGraph {
    public:
        typedef std::vector<int> VI;
        typedef std::vector<VI> Edges;
        typedef std::vector<Weight> Weights;
        typedef std::vector<Weights> EdgeWeights;
        typedef std::vector<VI> Matrix;
        typedef VI::iterator VertexIterator;
        
        Edges edges;
        EdgeWeights edge_weights;
        Weights vertex_weights;
        Matrix edge_exists;
        int num;
        
        void init( int n ) {
            num = n;
            edges.resize(num);
            vertex_weights.resize(num);
            edge_weights.resize(num);
            edge_exists.resize(num);
            std::fill( edges.begin(), edges.end(), VI() );
            std::fill( edge_weights.begin(), edge_weights.end(), Weights( num, 0 ) );
            std::fill( edge_exists.begin(), edge_exists.end(), VI( num, false ) );
            addVertices();
            addEdges();
        }
        void addEdge( int from, int to ) {
            if ( edge_exists[from][to] )
                return;
            edge_exists[from][to] = true;
            edges[from].push_back(to);
        }
        void addEdge( int from, int to, Weight weight ) {
            addEdge( from, to );
            setEdgeCost( from, to, weight );
        }
        int countVertices() {
            return num;
        }
        Weight getVertexCost( int from ) {
            return vertex_weights[from];
        }
        void setVertexCost( int from, Weight weight ) {
            vertex_weights[from] = weight;
        }
        Weight getEdgeCost( int from, int to ) {
            return edge_weights[from][to];
        }
        void setEdgeCost( int from, int to, Weight weight ) {
            edge_weights[from][to] = weight;
        }
        VertexIterator begin( int from ) {
            return edges[from].begin();
        }
        VertexIterator end( int from ) {
            return edges[from].end();
        }
    };
}

// @snippet<sh19910711/contest:graph/count_connected_vertices.cpp>
namespace graph {
    int CountConnectedVertices( graph::AdjacentListGraph& graph, int start_vertex ) {
        int n = graph.num;
        bool used[n];
        std::fill( used, used+n, false );
        std::queue <int> Q;
        Q.push(start_vertex);
        used[start_vertex] = true;
        int res = 1;
        while ( ! Q.empty() ) {
            int current_vertex = Q.front();
            Q.pop();
            for ( std::vector<int>::iterator it_i = graph.begin(current_vertex);
                    it_i != graph.end(current_vertex); ++ it_i ) {
                int& next_vertex = *it_i;
                if ( used[next_vertex] )
                    continue;
                used[next_vertex] = true;
                res ++;
                Q.push(next_vertex);
            }
        }
        return res;
    }
}

// @snippet<sh19910711/contest:solution/solution.cpp>
namespace solution {
    using namespace std;
    
    const int SIZE = 10001;
    int n;
    string A[SIZE];
    bool U[26];
    int vertices;
    
    class Graph: public graph::AdjacentListGraph {
    public:
        int findNode( char c ) {
            return c - 'a';
        }
        
        void init( int num ) {
            vertices = 0;
            graph::AdjacentListGraph::init(num);
        }
        
        graph::Weight& edge( int from, int to ) {
            return edge_weights[from][to];
        }
        
        void addVertices() {
            for ( int i = 0; i < n; ++ i ) {
                int a = findNode(*A[i].begin());
                int b = findNode(*A[i].rbegin());
                if ( a == b )
                    continue;
                if ( ! U[a] )
                    vertices ++;
                if ( ! U[b] )
                    vertices ++;
                U[a] = U[b] = true;
            }
        }
        
        void addEdges() {
            for ( int i = 0; i < n; ++ i ) {
                int a = findNode(*A[i].begin());
                int b = findNode(*A[i].rbegin());
                addEdge( a, b, 0 );
            }
            for ( int i = 0; i < n; ++ i ) {
                int a = findNode(*A[i].begin());
                int b = findNode(*A[i].rbegin());
                edge(a,b) ++;
            }
        }
    };
    
    class Solution: public ISolution {
    public:
        Graph G;
        void init() {
            fill( U, U+26, false );
        }
        bool input() {
            if ( ! ( cin >> n && n ) )
                return false;
            for ( int i = 0; i < n; ++ i )
                cin >> A[i];
            return true;
        }
        bool solve() {
            G.init(26);
            
            for ( int i = 0; i < 26; ++ i ) {
                if ( U[i] ) {
                    int ret = graph::CountConnectedVertices( G, i );
                    if ( ret != vertices )
                        return false;
                    break;
                }
            }
            
            for ( int i = 0; i < 26; ++ i ) {
                if ( ! U[i] )
                    continue;
                int cnt_in = 0;
                int cnt_out = 0;
                for ( int j = 0; j < 26; ++ j ) {
                    if ( i == j )
                        continue;
                    if ( ! U[j] )
                        continue;
                    cnt_in += G.edge( j, i );
                    cnt_out += G.edge( i, j );
                }
                if ( cnt_in != cnt_out )
                    return false;
            }
            
            return true;
        }
        void output( bool result ) {
            cout << ( result ? "OK" : "NG" ) << endl;
        }
        int run() {
            while ( init(), input() ) {
                output( solve() );
            }
            return 0;
        }
    };
}


// @snippet<sh19910711/contest:main.cpp>

int main() {
    return solution::Solution().run();
}

Measured memory_bytes: 2168