#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;

//template
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(a);i>(b);i--)
#define ALL(v) (v).begin(),(v).end()
typedef long long int ll; const int inf = 0x3fffffff; const ll INF = 0x3fffffffffffffff;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
//template end



int main(){
    int h,w; scanf("%d%d",&h,&w);
    vector<string> g(h);
    rep(i,0,h)cin>>g[i];
    bool f1=(w==1),f2=(h==1);
    rep(i,0,h)rep(j,0,w-1){
        if(g[i][j]!=g[i][j+1])break;
        if(j==w-2)f1=1;
    }
    rep(j,0,w)rep(i,0,h-1){
        if(g[i][j]!=g[i+1][j])break;
        if(i==h-2)f2=1;
    }
    if(f1&&f2)printf("NO\n"); else printf("YES\n");
    return 0;
}