import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int length = sc.nextInt();
        char[][] dnas = new char[n][];
        for (int i = 0; i < n; i++) {
            dnas[i] = sc.next().toCharArray();
        }
        PriorityQueue<Path> queue = new PriorityQueue<>();
        for (int i = 0; i < n - 1; i++) {
            for (int j = i + 1; j < n; j++) {
                int count = 0;
                for (int k = 0; k < length; k++) {
                    if (dnas[i][k] != dnas[j][k]) {
                        count++;
                    }
                }
                queue.add(new Path(count, i, j));
            }
        }
        int ans = 0;
        UnionFindTree uft = new UnionFindTree(n);
        while (queue.size() > 0) {
            Path p = queue.poll();
            if (uft.same(p)) {
                continue;
            }
            ans += p.cost;
            uft.unite(p);
        }
        System.out.println(ans);
    }
    
    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(Path p) {
            return same(p.left, p.right);
        }
        
        public void unite(int x, int y) {
            if (!same(x, y)) {
                parents[find(y)] = find(x);
            }
        }
        
        public void unite(Path p) {
            unite(p.left, p.right);
        }
    }
    
    static class Path implements Comparable<Path> {
        int cost;
        int left;
        int right;
        
        public Path(int cost, int left, int right) {
            this.cost = cost;
            this.left = left;
            this.right = right;
        }
        
        public int compareTo(Path another) {
            return cost - another.cost;
        }
    }
}

class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    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 String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}