#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int h, w; cin >> h >> w;
	vector<int>v(2);
	rrep(i, h) {
		string s; cin >> s;
		rep(j, w) {
			if ('o' == s[j])v[(i + j) % 2]++;
		}
	}
	int sum = v[0] + v[1];
	bool chk = false;
	if (0 == sum % 2) {
		int d = abs(v[0] - sum / 2);
		if (0 == d % 3)chk = true;
	}
	else {
		int d = abs(v[0] - (sum / 2 - 1));
		if (0 == d % 3)chk = true;
	}
	if (chk) cout << "YES" << endl;
	else cout << "NO" << endl;
	return 0;
}