#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> p64;
typedef vector<ll> v64;
typedef vector<v64> vv64;
typedef vector<p64> vp64;

int n, k;
bool adjmat[26][26];
int in[26];
int dp[26];
vector<int> graph[26];

int main() {
    cin >> n >> k;

    string s;
    for (int i=0; i<n; i++) {
        cin >> s;
        for (int j1=0; j1<k; j1++) {
            int a1 = s[j1] - 'A';
            for (ll j2=j1+1; j2<k; j2++) {
                int a2 = s[j2] - 'A';
                adjmat[a1][a2] = true;
            }
        }
    }

    for (int i=0; i<k; i++) {
        for (int j=i+1; j<k; j++) {
            if (adjmat[i][j] && adjmat[j][i]) {
                adjmat[i][j] = false;
                adjmat[j][i] = false;
            }
        }
    }

    for (int i=0; i<k; i++) for (int j=0; j<k; j++) {
        if (adjmat[i][j]) {
            graph[i].push_back(j);
            in[j]++;
        }
    }

    queue<int> q;
    for (int i=0; i<k; i++) {
        if (in[i] == 0) {
            q.push(i);
            dp[i] = 1;
        }
    }

    int maxlen = 0;
    while (!q.empty()) {
        int i = q.front();
        q.pop();

        maxlen = max(maxlen, dp[i]);

        for (int adj : graph[i]) {
            dp[adj] = dp[i]+1;
            q.push(adj);
        }
    }

    cout << maxlen << endl;

    return 0;
}