import std.stdio, std.array, std.string, std.conv, std.algorithm;
import std.typecons, std.range, std.random, std.math, std.container;
import std.numeric, std.bigint, core.bitop, core.stdc.string;

void main() {
    auto s = readln.split.map!(to!int);
    auto H = s[0];
    auto W = s[1];
    auto A = H.iota.map!(_ => readln.chomp).array;

    Tuple!(int, int)[] B;
    foreach (i; 0..H) foreach (j; 0..W) if (A[i][j] == 'o') B ~= tuple(i, j);

    auto cnt = new int[](2);
    foreach (i; 0..B.length) cnt[(B[i][0] + B[i][1]) % 2] += 1;

    if (B.length == 0) {
        writeln("YES");
    } else if (B.length == 1) {
        writeln("NO");
    } else if (B.length == 2) {
        writeln(cnt[0] == 1 ? "YES" : "NO");
    } else if (B.length == 3) {
        assert(false);
    } else {
        writeln("YES");
    }
}