import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int length = sc.nextInt();
        char[][] all = new char[n][];
        for (int i = 0; i < n; i++) {
            all[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++) {
                queue.add(new Path(i, j, getCost(all[i], all[j])));
            }
        }
        UnionFindTree uft = new UnionFindTree(n);
        long cost = 0;
        while (queue.size() > 0) {
            Path p = queue.poll();
            if (!uft.same(p)) {
                uft.unite(p);
                cost += p.value;
            }
        }
        System.out.println(cost);
    }
    
    static int getCost(char[] a1, char[] a2) {
        int cost = 0;
        for (int i = 0; i < a1.length; i++) {
            if (a1[i] != a2[i]) {
                cost++;
            }
        }
        return cost;
    }
    
    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);
        }
    }
    
    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;
        }
    }
}