#include <algorithm>
#include <iostream>

int main() {
	using namespace std;
	int n, m;
	cin >> n >> m;
	unsigned cnt[128]{};
	for(int i = 0; i < n; i++)
	{
		char ch;
		cin >> ch;
		++cnt[ch];
	}
	unsigned ans = 0;
	for(int i_d = 0; i_d <= m; i_d++)
		for(int i_k = 0; i_k <= m - i_d; i_k++)
			for(int i_o = 0; i_o <= m - i_d - i_k; i_o++)
				for(int i_r = 0; i_r <= m - i_d - i_k - i_o; i_r++)
				{
					cnt['d'] += i_d;
					cnt['k'] += i_k;
					cnt['o'] += i_o;
					cnt['r'] += i_r;
					ans = max(ans, min({cnt['k'] / 2, cnt['o'] / 2, cnt['d'], cnt['r']}) * 7 + 
						cnt['d'] * cnt['d'] + cnt['k'] * cnt['k'] + cnt['o'] * cnt['o'] + cnt['r'] * cnt['r']);
					cnt['d'] -= i_d;
					cnt['k'] -= i_k;
					cnt['o'] -= i_o;
					cnt['r'] -= i_r;
				}
	cout << ans << endl;
	return 0;
}