#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
typedef long long ll;
typedef long double ld;
using namespace std;
const int N = 1e5 + 5;
int n, k;
int pos[26];
int get(vector<int> &seq, int high, int x) {
	int low = 0, mid, j;
	while(low <= high) {
		mid = low + high >> 1;
		if(seq[mid] >= x)
			high = mid - 1, j = mid;
		else
			low = mid + 1;
	}
	return j;
}
int main() {
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    cin >> n >> k;
    for(int i = 0;i < n;i++) {
		string s;
		cin >> s;
		for(int j = 0;j < k;j++) {
			int cur = s[j] - 'A';
			pos[cur] = max(pos[cur], j);
		}
	}
	int len = 1;
	vector<int> seq(k, 0);
	seq[0] = pos[0];
	for(int i = 1;i < k;i++) {
		if(pos[i] < seq[0])
			seq[0] = pos[i];
		else if(pos[i] > seq[len - 1])
			seq[len++] = pos[i];
		else {
			int j = get(seq, len - 1, pos[i]);
			seq[j] = pos[i];
		}
	}
	cout << len;
    return 0;
}