#include<bits/stdc++.h>
using namespace std;
map<char,int> mp;
long long ans,sum;
int n,m;
char l[]={'k','r','o','d'};
void dfs(int pos)
{
	if(pos==m+1)
	{
		sum=0;
		sum+=mp['d']*mp['d']+mp['k']*mp['k']+mp['o']*mp['o']+mp['r']*mp['r'];
		sum+=min({mp['k']/2,mp['o']/2,mp['r'],mp['d']})*7;
		ans=max(ans,sum);
		return ;
	}
	for(int j=0;j<4;j++)
	{
		mp[l[j]]++;
		dfs(pos+1);
		mp[l[j]]--;
	}
}
int main()
{
	cin>>n>>m;
	string s;
	cin>>s;
	for(int i=0;i<s.length();i++)
		mp[s[i]]++;
	dfs(1);
	cout<<ans;
	return 0;
}