/* -*- coding: utf-8 -*-
 *
 * 1166.cc:  No.1166 NADA DNA - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 1000;
const int MAX_M = MAX_N * (MAX_N - 1) / 2;
const int MAX_L = 50;

/* typedef */

struct Edge {
  int d, i, j;
  Edge() {}
  Edge(int _d, int _i, int _j): d(_d), i(_i), j(_j) {}
  bool operator<(const Edge &e) const { return d < e.d; }
};

struct UFT {
  int links[MAX_N], ranks[MAX_N], sizes[MAX_N];
  UFT() {}

  void init(int n) {
    for (int i = 0; i < n; i++) links[i] = i, ranks[i] = sizes[i] = 1;
  }

  int root(int i) {
    int i0 = i;
    while (links[i0] != i0) i0 = links[i0];
    return (links[i] = i0);
  }

  int rank(int i) { return ranks[root(i)]; }
  int size(int i) { return sizes[root(i)]; }
  bool same(int i, int j) { return root(i) == root(j); }

  int merge(int i0, int i1) {
    int r0 = root(i0), r1 = root(i1), mr;
    if (r0 == r1) return r0;
    if (ranks[r0] == ranks[r1]) {
      links[r1] = r0;
      sizes[r0] += sizes[r1];
      ranks[r0]++;
      mr = r0;
    }
    else if (ranks[r0] > ranks[r1]) {
      links[r1] = r0;
      sizes[r0] += sizes[r1];
      mr = r0;
    }
    else {
      links[r0] = r1;
      sizes[r1] += sizes[r0];
      mr = r1;
    }
    return mr;
  }
};

/* global variables */

char gs[MAX_N][MAX_L + 4];
Edge es[MAX_M];
UFT uft;

/* subroutines */

inline int hdist(char s[], char t[], int l) {
  int d = 0;
  for (int i = 0; i < l; i++)
    if (s[i] != t[i]) d++;
  return d;
}

/* main */

int main() {
  int n, l;
  scanf("%d%d", &n, &l);
  for (int i = 0; i < n; i++) scanf("%s", gs[i]);

  int m = 0;
  for (int i = 0; i < n; i++)
    for (int j = i + 1; j < n; j++)
      es[m++] = Edge(hdist(gs[i], gs[j], l), i, j);
  sort(es, es + m);

  uft.init(n);

  int sum = 0;
  for (int i = 0; i < m; i++) {
    Edge &e = es[i];
    if (! uft.same(e.i, e.j)) {
      uft.merge(e.i, e.j);
      sum += e.d;
    }
  }

  printf("%d\n", sum);
  return 0;
}