class UnionFind:
  def __init__(self, n):
    self.node = [-1 for _ in range(n)]

  def root(self, v):
    if self.node[v] < 0:
      return v

    st = []
    while self.node[v] >= 0:
      st.append(v)
      v = self.node[v]

    for u in st:
      self.node[u] = v

    return v

  def size(self, v):
    v = self.root(v)
    return (- self.node[v])

  def same(self, u, v):
    return self.root(u) == self.root(v)

  def unite(self, u, v):
    ru = self.root(u)
    rv = self.root(v)
    if ru == rv:
      return

    du = self.node[ru]
    dv = self.node[rv]
    if du <= dv:
      self.node[rv] = ru
      self.node[ru] += dv
    else:
      self.node[ru] = rv
      self.node[rv] += du

n, l = map(int, input().split())
s = [input() for _ in range(n)]

edges = []
for u in range(n - 1):
  for v in range(u + 1, n):
    cost = 0
    for i in range(l):
      if s[u][i] != s[v][i]:
        cost += 1

    edges.append((u, v, cost))

edges.sort(key=lambda x: x[2])
ans = 0
uf = UnionFind(n)
for u, v, w in edges:
  if not uf.same(u, v):
    uf.unite(u, v)
    ans += w

print(ans)