/* -*- 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 */

/* global variables */

char gs[MAX_N][MAX_L + 4];
int ds[MAX_M];

/* 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++)
      ds[m++] = hdist(gs[i], gs[j], l);
  sort(ds, ds + m);

  int sum = 0;
  for (int i = 0; i < n - 1; i++) sum += ds[i];

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