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

typedef long long ll;
typedef pair<int,int> P;

#define p_ary(ary,a,b) do { cout << "["; for (int count = (a);count < (b);++count) cout << ary[count] << ((b)-1 == count ? "" : ", "); cout << "]\n"; } while(0)
#define p_map(map,it) do {cout << "{";for (auto (it) = map.begin();;++(it)) {if ((it) == map.end()) {cout << "}\n";break;}else cout << "" << (it)->first << "=>" << (it)->second << ", ";}}while(0)

int main() {
    int h,w;
    cin >> h >> w;
    vector<string> a(h);
    for (int i = 0;i < h;++i) cin >> a[i];
    bool tate = false,yoko = false;
    for (int i = 0;i < h;++i) {
        bool same = true;
        for (int j = 1;j < w;++j) {
            if (a[i][j-1] != a[i][j]) {
                same = false;
                break;
            }
        }
        if (same) yoko = true;
    }
    for (int i = 0;i < w;++i) {
        bool same = true;
        for (int j = 1;j < h;++j) {
            if (a[j-1][i] != a[j][i]) {
                same = false;
                break;
            }
        }
        if (same) tate = true;
    }
    cout << (tate && yoko ? "NO" : "YES") << endl;
    return 0;
}