#include<bits/stdc++.h>
//#define int long lnog
using namespace std;
string s,t;
int ans,n,m;
void dfs(int res){
	if(res>=m){
		map<char,int> mp;
		for(int i=0;i<s.size();i++){
			mp[s[i]]++;
		}
		for(int i=0;i<m;i++){
			mp[t[i]]++;
		}
		ans=max(ans,mp['d']*mp['d']+mp['k']*mp['k']+mp['o']*mp['o']+mp['r']*mp['r']+7*min({mp['d'],mp['k']/2,mp['o']/2,mp['r']}));
		return;
	}
	t[res]='d';
	dfs(res+1);
	t[res]='k';
	dfs(res+1);
	t[res]='o';
	dfs(res+1);
	t[res]='r';
	dfs(res+1);
}
int main(){
	cin>>n>>m>>s;
//	long long n;
//	cin>>n;
//	int ans=max(1,n);
	for(int i=1;i<=m;i++)t+=' ';
	
	dfs(0);
	cout<<ans;
}