#include <iostream>
#include <set>
#include <string>
#include <vector>
using namespace std;


int main() {
    int N, M;
    cin >> N >> M;
    vector<string> S(N, string(M, '.'));
    for (int i = 0; i < M; ++i) {
        for (auto &s : S) cin >> s.at(i);
    }
    cout << (S.size() == set<string>{S.begin(), S.end()}.size() ? "Yes" : "No") << '\n';
}