// Author: Ivan Kazmenko (gassa@mail.ru)
// construct a chain of observations by repeatedly finding the closest one
module solution;
import std.algorithm, std.array, std.conv, std.stdio, std.string;

immutable int infinity = int.max / 2;

int dist (int [] u, int [] v)
{
	int res = infinity;
	foreach (i; 0..u.length)
		if (u[i] && v[i] && u[i] > v[i])
			res = min (res, u[i] - v[i]);
	return res;
}

int solve (int n, int m, int [] [] a)
{
	int [] cur = a[0];
	int res = 0;
	do
	{
		int [] next = a.minElement !(p => dist (cur, p));
		int delta = dist (cur, next);
		if (delta == infinity)
			return -1;
		res += delta;
		cur = next;
	}
	while (cur != a[0]);
	return res;
}

void main ()
{
	int n, m;
	readf (" %s %s", &n, &m);
	readln;
	int [] [] a;
	foreach (k; 0..m)
		a ~= readln.split.map !(x => x == "X" ? 0 : x.to!int).array;
	writeln (solve (n, m, a));
}