#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {
    int n, l;
    cin >> n >> l;
    vector<string> usernames;
    
    for(int i=0;i<n;i++) {
        string s;
        cin >> s;
        usernames.push_back(s);
    }

    unsigned long long sum = 0;
    for(int i=0;i<n;i++) {
        string user = usernames[i];
        int closest_match = 0;
        for(int j=0;j<n;j++) {
            if(i==j) {
                continue;
            }
            string other_user = usernames[j];
            for(int k=0;k<l;k++) {
                if(user[k] != other_user[k]) {
                    closest_match = max(closest_match, k+1);
                    break;
                }
            }
        }
	sum += closest_match;
    }
    cout << sum << endl;
    return 0;
}