#include<bits/stdc++.h>
#include<map>
#define int long long
#define fi first
#define se second
#define F(i,l,r) for(int i=(l); i<=(r); i++)
using namespace std;
const int N= 1e6 +50, M= 1e3 +50;
const int INF=0x3f3f3f3f3f3f3f3f, mod=1e9+7;

int n,m,ans;
int mp[10];

inline void chk(){
	int res = 0, mi=INF;
	F(i,1,4){
		if(i<3) mi = min(mi, mp[i]/2);
		else mi = min(mi,mp[i]);
		res += mp[i]*mp[i];
	}
	ans = max(ans,res+mi*7);
}
inline void DFS(int pos,int c){
	if(pos>4){
		chk();
		return ;
	}
	F(i,0,c){
		mp[pos]+=i;
		DFS(pos+1,c-i);
		mp[pos]-=i;
	}
}

signed main(){
	ios::sync_with_stdio(false);
	cin.tie(0),cout.tie(0);
	
	cin>>n>>m;
	F(i,1,n){
		char c; cin>>c;
		if(c=='k') mp[1]++;
		if(c=='o') mp[2]++;
		if(c=='d') mp[3]++;
		if(c=='r') mp[4]++;
	}
	DFS(1,m);
	cout<<ans<<'\n';
	
	return 0;
}