#include <iostream>
using namespace std;

string mat[111];
int main(){
    int H, W;
    cin >> H >> W;
    for(int y=0;y<H;y++)cin >> mat[y];
    
    int top = 111, bot = -1, lef = 111, rig = -1;
    int tot = 0;
    
    for(int y=0;y<H;y++){
        for(int x=0;x<W;x++)if(mat[y][x] == '.'){
            top = min(top, y);
            bot = max(bot, y);
            lef = min(lef, x);
            rig = max(rig, x);
            
            ++tot;
        }
    }
    
    int kado = 0;
    if(mat[top][lef] == '.')++kado;
    if(mat[top][rig] == '.')++kado;
    if(mat[bot][lef] == '.')++kado;
    if(mat[bot][rig] == '.')++kado;
    
    if(kado >= 3 && bot - top >= 1 && rig - lef >= 1){
        int cnt = (bot - top + 1) * 2 + (rig - lef + 1) * 2 - 4;
        if(cnt == tot){
            cout << "YES" << endl;
            return 0;
        }
    }
    
    cout << "NO" << endl;
    return 0;
}