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


def distant(a, b):
    c = 0
    for i, j in zip(a, b):
        c += i != j
    return c


class UnionFind:
    def __init__(self, n):
        self.parent = [-1] * n
        self.rank = [1] * n

    def get_root(self, i):
        while self.parent[i] != -1:
            i = self.parent[i]
        return i

    def is_same_tree(self, i, j):
        return self.get_root(i) == self.get_root(j)

    def merge(self, i, j):
        i = self.get_root(i)
        j = self.get_root(j)
        if i == j:
            return

        if self.rank[i] > self.rank[j]:
            self.parent[j] = i
            self.rank[i] = max(self.rank[i], self.rank[j] + 1)
        else:
            self.parent[i] = j
            self.rank[j] = max(self.rank[j], self.rank[i] + 1)


uf = UnionFind(n)
cost_vec = [(distant(g[i], g[j]), i, j)
            for i in range(n) for j in range(i + 1, n)]
cost_vec.sort()

cost = 0
index = 0
for _ in range(n - 1):
    while True:
        c, i, j = cost_vec[index]
        index += 1
        if not uf.is_same_tree(i, j):
            break

    cost += c
    uf.merge(i, j)
print(cost)