#define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define MT make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define RT return
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;

void solve() {
    int H, W;
    cin >> H >> W;
    vi cnt(2);
    rep(y, H) {
        string S;
        cin >> S;
        rep(x, W) {
            if (S[x] == 'o') {
                cnt[(x + y) % 2]++;
            }
        }
    }

    int p = (H - 1) % 2;
    int m3 = (cnt[p] + cnt[1 ^ p]*2) % 3;
    cout << (m3 ? "NO" : "YES") << endl;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(15);
	solve();
	return 0;
}