# https://yukicoder.me/problems/no/1166


class UnionFind:
    """
    UnionFindの基本的な処理を実装したクラス
    """

    def __init__(self, size):
        self.root = [i for i in range(size)]
        self.size = [1] * size

    def get_root(self, v):
        if v == self.root[v]:
            return v
        else:
            old_root = self.root[v]
            new_root = self.get_root(old_root)
            self.root[v] = new_root
            return new_root

    def merge(self, u, v):
        root_u = self.get_root(u)
        root_v = self.get_root(v)
        if root_u == root_v:
            return False

        if self.size[root_u] >= self.size[root_v]:
            self.size[root_u] += self.size[root_v]
            self.root[root_v] = root_u
            self.root[v] = root_u
        else:
            self.size[root_v] += self.size[root_u]
            self.root[root_u] = root_v
            self.root[u] = root_v
        return True
    

def main():
    N, L = map(int, input().split())
    G = []
    for _ in range(N):
        G.append(input())

    edges = []
    for i in range(N):
        for j in range(i + 1, N):
            a = 0
            for k in range(L):
                if G[i][k] != G[j][k]:
                    a += 1
            edges.append((i, j, a))
    
    edges.sort(key=lambda x : x[2])
    answer = 0
    uf = UnionFind(N)
    for i, j, a in edges:
        if uf.merge(i, j):
            answer += a
    print(answer)







if __name__ == "__main__":
    main()