#include<bits/stdc++.h>
using namespace std;
#define int long long
int a[5],n,m,maxx=-1e9;
string s;
void dfs(int x){
	if(x>m){
		maxx=max(maxx,a[1]*a[1]+a[2]*a[2]+a[3]*a[3]+a[4]*a[4]+min(min(a[1],a[2]/2),min(a[3]/2,a[4]))*7);
		return ;
	}
	for(int i=1;i<=4;i++){
		a[i]++;
		dfs(x+1);
		a[i]--;
	}
	return ;
}
signed main(){
	cin>>n>>m>>s;
	for(int i=0;i<n;i++){
		if(s[i]=='d') a[1]++;
		else if(s[i]=='k') a[2]++;
		else if(s[i]=='o') a[3]++;
		else a[4]++;
	}
	dfs(1);
	cout<<maxx;
		
	
	return 0;
}//d k o r