int n, m, l, a[499500], b[499500], c[499500], z;
char s[1d3][51];
wgraph<int> g;
{
	rd(n, l, s(n));
	m = n * (n - 1) / 2;
	rep(i, n) rep(j, i + 1, n) {
		c[z] += s[i][0..l] != s[j][0..l];
		a[z] = i;
		b[z] = j;
		z++;
	}
	g.setEdge(n, m, a, b, c);
	wt(g.MST_Prim_cost());
}