sub x{exit!print"No"}
($h,$w)=glob<>;
@s=<>;
($s[0]^$s[$_])=~/^(.)\1*.$/||x for 1..$h-1;
$t[$`][$']=/ / for@s[$h+1..$#s];
$t[1][$_]|$s[$_-1]ne$s[$_]||x for 1..$h-1;
$t[2][$_]|vec($s[0],$_-1,8)-vec($s[0],$_,8)||x for 1..$w-1;
print"Yes";