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<Bridge> 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<Bridge> {
        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();
    }
    
}