#include<bits/stdc++.h>
using namespace std;
int t[256];
int calc(int k,int o,int r,int d){
	int ks=min({k/2,o/2,d,r});
	return k*k+o*o+r*r+d*d+ks*7;
}
int main(){
	int n,m;
	cin>>n>>m;
	char c;
	for(int i=1;i<=n;i++){
		cin>>c;
		t[c]++;
	}
	n+=m;
	int ans=-1;
	for(int cntk=t['k'];cntk<=n;cntk++){
		for(int cnto=t['o'];cnto<=n-cntk;cnto++){
			for(int cntr=t['r'];cntr<=n-cntk-cnto;cntr++){
				for(int cntd=t['d'];cntd<=n-cntk-cnto-cntr;cntd++){
					ans=max(ans,calc(cntk,cnto,cntr,cntd));
				}
			}
		}
	}
	cout<<ans;
	return 0;
}