import java.io.*; import java.util.*; import java.util.stream.*; public class Main { public static void main(String[] args) throws Exception { Scanner sc = new Scanner(); int n = sc.nextInt(); int length = sc.nextInt(); char[][] inputs = new char[n][]; for (int i = 0; i < n; i++) { inputs[i] = sc.next().toCharArray(); } ArrayList bridges = new ArrayList<>(); for (int i = 0; i < n; i++) { for (int j = i + 1; j < n; j++) { int count = 0; for (int k = 0; k < length; k++) { if (inputs[i][k] != inputs[j][k]) { count++; } } bridges.add(new Bridge(i, j, count)); } } Collections.sort(bridges); long ans = 0; UnionFindTree uft = new UnionFindTree(n); for (Bridge x : bridges) { if (!uft.same(x)) { uft.unite(x); ans += x.value; } } System.out.println(ans); } static class Bridge implements Comparable { int left; int right; int value; public Bridge(int left, int right, int value) { this.left = left; this.right = right; this.value = value; } public int compareTo(Bridge another) { return value - another.value; } } static class UnionFindTree { int[] parents; public UnionFindTree(int size) { parents = new int[size]; for (int i = 0; i < size; i++) { parents[i] = i; } } public int find(int x) { if (x == parents[x]) { return x; } else { return parents[x] = find(parents[x]); } } public boolean same(int x, int y) { return find(x) == find(y); } public boolean same(Bridge x) { return same(x.left, x.right); } public void unite(int x, int y) { parents[find(x)] = find(y); } public void unite(Bridge x) { unite(x.left, x.right); } } } class Utilities { static String arrayToLineString(Object[] arr) { return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n")); } static String arrayToLineString(int[] arr) { return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new)); } } class Scanner { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); StringTokenizer st = new StringTokenizer(""); StringBuilder sb = new StringBuilder(); public Scanner() throws Exception { } public int nextInt() throws Exception { return Integer.parseInt(next()); } public long nextLong() throws Exception { return Long.parseLong(next()); } public double nextDouble() throws Exception { return Double.parseDouble(next()); } public int[] nextIntArray() throws Exception { return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray(); } public String next() throws Exception { while (!st.hasMoreTokens()) { st = new StringTokenizer(br.readLine()); } return st.nextToken(); } }