#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;
#define ll long long
template<typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

typedef long double ld;


#define lmao ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

const ll N = 1e5 + 5, M = 1e4 + 5, MOD = 998244353, LOG = 20, Msk = 21, pw1 = 31, pwe2 = 37;

int n , k;
int occur[28][N];
ll dp[28];

ll slv(ll last , int cnt = 0){
    if(cnt == k)
        return 0;
    ll &ret = dp[last];
    if(~ret)
        return ret;
    ret = 0;
    for(int i = 0  ; i < k ; i++){
        bool can = true;
        if(last == i)
            continue;
        for(int j = 0 ; j < n ; j++){
            if(occur[i][j] < occur[last][j])
                can = false;
        }
        if(can)
            ret = max(ret , slv(i , cnt + 1) + 1);
    }
    return ret;
}

void testCase() {
    cin >> n >> k;
    vector<string> v(n);
    for (int i = 0; i < n; i++)
        cin >> v[i];

    for(int i = 0 ; i < n ; i++){
        for(int j = 0 ; j < k ; j++)
            occur[v[i][j] -'A'][i] = j;
    }
    memset(dp,-1,sizeof dp);
    cout << slv(27);
}

signed main() {

    lmao
    int t = 1;
//    cin >> t;
    while (t--)
        testCase();
}