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[][] inputs = new char[n][];
        ArrayList<Path> list = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            inputs[i] = sc.next().toCharArray();
            for (int j = 0; j < i; j++) {
                list.add(new Path(i, j, getDistance(inputs[i], inputs[j])));
            }
        }
        Collections.sort(list);
        UnionFindTree uft = new UnionFindTree(n);
        int ans = 0;
        for (Path p : list) {
            if (!uft.same(p)) {
                ans += p.value;
                uft.unite(p);
            }
        }
        System.out.println(ans);
    }
    
    static int getDistance(char[] s, char[] t) {
        int distance = 0;
        for (int i = 0; i < s.length; i++) {
            if (s[i] != t[i]) {
                distance++;
            }
        }
        return distance;
    }
    
    static class Path implements Comparable<Path> {
        int left;
        int right;
        int value;
        
        public Path(int left, int right, int value) {
            this.left = left;
            this.right = right;
            this.value = value;
        }
        
        public int compareTo(Path 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(Path p) {
            return same(p.left, p.right);
        }
        
        public void unite(int x, int y) {
            parents[find(x)] = find(y);
        }
        
        public void unite(Path p) {
            unite(p.left, p.right);
        }
    }
    
}
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 String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}