#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define srep(i,s,t) for(int i=(int)(s);i<(int)(t);++i)
#define each(a,b) for(auto& (a): (b))
#define all(v) (v).begin(),(v).end()
#define len(v) (int)(v).size()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define cmx(x,y) x=max(x,y)
#define cmn(x,y) x=min(x,y)
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define sar(a,n) cout<<#a<<":";rep(pachico,n)cout<<" "<<a[pachico];cout<<endl

using namespace std;

template<typename S,typename T>auto&operator<<(auto&o,pair<S,T>p){return o<<"{"<<p.fi<<","<<p.se<<"}";}
template<typename T>auto&operator<<(auto&o,set<T>s){for(auto&e:s)o<<e<<" ";return o;}
template<typename S,typename T,typename U>
auto&operator<<(auto&o,priority_queue<S,T,U>q){while(!q.empty())o<<q.top()<<" ",q.pop();return o;}
template<typename K,typename T>auto&operator<<(auto&o,map<K,T>&m){for(auto&e:m)o<<e<<" ";return o;}
template<typename T>auto&operator<<(auto&o,vector<T>v){for(auto&e:v)o<<e<<" ";return o;}
void ashow(){cout<<endl;}template<typename T,typename...A>void ashow(T t,A...a){cout<<t<<" ";ashow(a...);}
template<typename S,typename T,typename U>
struct TRI{S fi;T se;U th;TRI(){}TRI(S f,T s,U t):fi(f),se(s),th(t){}};
template<typename S,typename T,typename U>
auto& operator<<(auto& o,TRI<S,T,U>& t){return o<<"{"<<t.fi<<","<<t.se<<","<<t.th<<"}";}

typedef pair<int, int> P;
typedef pair<ll, ll> pll;
typedef TRI<int, int, int> tri;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef vector<P> vp;
typedef vector<double> vd;
typedef vector<string> vs;

const int MAX_N = 13;

int h, w;
int id[MAX_N], ok[MAX_N], used[MAX_N];
char c[MAX_N][MAX_N];
string s[MAX_N], t[MAX_N];

bool dfs(int i, int d, int hoge)
{
    if(d == h/2){
        rep(i,h) ok[i] = 0;
        int nw = 0;
        // sar(id, h);
        rep(i,h){
            if(!ok[i]){
                if(id[i] < 0){
                    rep(j,w){
                        c[h/2][j] = s[i][j];
                    }
                }else{
                    rep(j,w){
                        c[nw][j] = s[i][j];
                        c[h-1-nw][j] = s[id[i]][j];
                    }
                    nw++;
                }
                ok[i] = ok[id[i]] = 1;
            }
        }
        rep(i,w){
            t[i].clear();
            rep(j,h){
                t[i].push_back(c[j][i]);
            }
        }
        rep(i,w) used[i] = 0;
        if(w % 2){
            bool mid = false;
            rep(i,w){
                if(!used[i]){
                    bool bue = false;
                    reverse(all(t[i]));
                    for(int j = i+1; j < w; j++){
                        if(t[i] == t[j]){
                            used[i] = used[j] = 1;
                            bue = true;
                            break;
                        }
                    }
                    if(!bue){
                        bool ch = false;
                        rep(j,h){
                            if(t[i][j] != t[i][h-1-j]){
                                ch = true;
                                break;
                            }
                        }
                        if(ch){
                            return false;
                        }else if(mid){
                            return false;
                        }else{
                            mid = true;
                            used[i] = 1;
                        }
                    }
                }
            }
            return true;
        }else{
            rep(i,w){
                if(!used[i]){
                    bool bue = false;
                    reverse(all(t[i]));
                    for(int j = i+1; j < w; j++){
                        if(t[i] == t[j]){
                            used[i] = used[j] = 1;
                            bue = true;
                            break;
                        }
                    }
                    if(!bue) return false;
                }
            }
            return true;
        }
    }else{
        for(;i < h; i++){
            if(id[i] < 0){
                for(int j = i+1; j < h; j++){
                    if(id[j] < 0){
                        id[i] = j, id[j] = i;
                        if(dfs(i+1, d+1, hoge)) return true;
                        id[i] = -1, id[j] = -1;
                    }
                }
                if(h % 2 == 0 || hoge) return false;
                hoge = 1;
            }
        }
    }
    return false;
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> h >> w;
    rep(i,h){
        cin >> s[i];
        id[i] = -1;
    }
    if(dfs(0, 0, 0)){
        cout << "YES\n";
    }else{
        cout << "NO\n";
    }
    return 0;
}


Measured memory_bytes: 256