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

public class NickyEnMarkBufferedReader{
    public static void main(String[] args) throws IOException {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String[] line = reader.readLine().split(" ");
            int n = Integer.parseInt(line[0]);
            int l = Integer.parseInt(line[1]);
            String[] names = new String[n];
            for(int i=0;i<n;i++){
                names[i]=reader.readLine();
            }
            Arrays.sort(names);
            int answer=0;
            for(int i=1;i<n-1;i++){
                int tempanswer=0;
                int tempanswer2=0;
                for(int j=0;j<l;j++){
                    if(names[i].charAt(j)!=names[i-1].charAt(j)){
                        tempanswer = j + 1;
                        break;
                    }
                }
                for(int j=0;j<l;j++){
                    if(names[i].charAt(j)!=names[i+1].charAt(j)){
                        tempanswer2 = j + 1;
                        break;
                    }
                }
                if(tempanswer>tempanswer2){
                    answer+=tempanswer;
                }
                else{
                    answer+=tempanswer2;
                }
            }
            for(int i=0;i<l;i++){
                if(names[0].charAt(i)!=names[1].charAt(i)){
                    answer = answer + i + 1;
                    break;
                }
            }
            for(int i=0;i<l;i++){
                if(names[n-2].charAt(i)!=names[n-1].charAt(i)){
                    answer = answer + i + 1;
                    break;
                }
            }
            System.out.println(answer);
        }
    }
}