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

int main()
{
    int H, W; cin>>H>>W;
    vector<string> S(H); for(auto& e: S) cin>>e;

#define REP(i, a, b) for(int i=a; i<b; ++i)
    
    REP(r, 0, H) REP(c, 0, W) if (c|r) {
        auto T=S;
        REP(i, 0, H-r) REP(j, 0, W-c) 
            if (T[i][j] && S[i][j]==S[i+r][j+c]) T[i][j]=T[i+r][j+c]=0;

        int no=0;
        REP(i, 0, H) REP(j, 0, W)
            no|=T[i][j] && S[i][j]=='#';
        if (!no) return !printf("YES\n");
    }
    puts("NO");
}