#include<iostream>
#include<fstream>
#include<sstream>
#include<string>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<stack>
#include<queue>
#include<set>
#include<map>
#include<vector>
#include<list>
#include<algorithm>
#include<utility>
#include<complex>

using namespace std;

#define reE(i,a,b) for(auto (i)=(a);(i)<=(b);(i)++)
#define rE(i,b) reE(i,0,b)
#define reT(i,a,b) for(auto (i)=(a);(i)<(b);(i)++)
#define rT(i,b) reT(i,0,b)
#define rep(i,a,b) reE(i,a,b);
#define rev(i,a,b) for(auto (i)=(b)-1;(i)>=(a);(i)--)
#define fe(i,b) for (auto &(x):b);
#define itr(i,b) for(auto (i)=(b).begin();(i)!=(b).end();++(i))
#define rti(i,b) for(auto (i)=(b).rbegin();(i)!=(b).rend();++(i))
#define LL long long


int bw[100][100];
int c[100][100];
string str;
int h, w;
bool board(){
	int r = 0, b = 0;
	rT(i, h)
		rT(j, w){
		if (c[i][j] == 1)r++;
		else if (c[i][j] == 2)b ++;
		c[i][j] = bw[i][j];
	}
	return (r>0&&b>0&&r==b);
}

int main(void){
	cin >> h >> w;
	int t=0;
	rT(i, h){
		cin >> str;
		rT(j, w){
			if (str[j] == '.')bw[i][j] = -1;
			else bw[i][j] = 0,t++;
		}
	}
	if (t == 0){
		cout << "YES" << endl;
		return 0;
	}
	board();
	rT(i,h)
		rT(j, w){
		if (i == 0 && j == 0)continue;
		bool flag = true;
		rT(x, h)
			rT(y, w){
			if (c[x][y] == 0){
				c[x][y] = 1;
				if (x + i >= h || y + j >= w )
					flag = false;
				else if (c[x + i][y + j] != 0)
					flag = false;
				else
				c[x + i][y + j] = 2;
			}
		}
		flag &= board();
		if (flag){
			cout << "YES" << endl;
			return 0;
		}
	}
	cout << "NO" << endl;
	return(0);
}