($m,$n)=glob<>;
push@a,[glob]for<>;
$b=99;

sub d {
	my($y,$x)=@_;
	for$j($y-1..$y+1){
		if($j>=0&&$j<$m){
			for$i($x-1..$x+1){
				if($i>=0&&$i<$n){
					$a[$j][$i]^=1;
				}
			}
		}
	}
}

sub f {
	my($y,$x,$c)=@_;
	
	if($x==$n){
		$x=0;++$y;
	}
	if($y==$m){
		for$j(0..$m-1){
			for$i(0..$n-1){
				if($a[$j][$i]){
					return;
				}
			}
		}
		if($b>$c){
			$b=$c;
		}
		return;
	}

	if($y>0&&$x>0){
		if($a[$y-1][$x-1]){
			d($y,$x);
			f($y,$x+1,$c+1);
			d($y,$x);
		}else{
			f($y,$x+1,$c);
		}
	}else{
		f($y,$x+1,$c);
		d($y,$x);
		f($y,$x+1,$c+1);
		d($y,$x);
	}
}		

f(0,0,0);
print $b;