#include<bits/stdc++.h>
#define int long long
#define rep(i,a,n) for(int i=1; i<=n; i++)
#define cios cin.tie(0),cout.tie(0),ios::sync_with_stdio(0);
using namespace std;
int n,m,d,k,o,r,cnt,a[4];
string s;
signed main(){
	cios;cin>>n>>m>>s;s=" "+s;
	rep(i,1,s.size()){
		if(s[i]=='d') d++;
		if(s[i]=='k') k++;
		if(s[i]=='o') o++;
		if(s[i]=='r') r++;
	}
	if(!m){
		cnt=min(d,min(k/2,min(o/2,r)))*7;
		cout<<cnt+d*d+k*k+o*o+r*r;
	}else if(m<=2){
		int d1=d,k1=k,o1=o,r1=r,m1=0;
		int cd=0,ck=0,co=0,cr=0;
		while(1){
			m1=m;cd=0,ck=0,co=0,cr=0;
			if(!d1) m--,d++,cd=1;
			else d1--;
			if(!r1) m--,r++,cr=1;
			else r1--;
			if(k1==1) m--,k1--,k++,ck=1;
			else if(!k1) m-=2,k+=2,ck=2;
			else k1-=2;
			if(o1==1) m--,o1--,o++,co=1;
			else if(!o1) m-=2,o+=2,co=2;
			else o1-=2;
			if(m<0){m=m1;break;}
			cnt++;
			if(!m){
				cd=0,ck=0,co=0,cr=0;
				break;
			}
		}
		a[0]=d-cd,a[1]=k-ck,a[2]=r-cr,a[3]=o-co;
		sort(a,a+4);
		cout<<cnt*7+a[0]*a[0]+a[1]*a[1]+a[2]*a[2]+(a[3]+m)*(a[3]+m);
	}else{
		a[0]=d,a[1]=k,a[2]=r,a[3]=o;
		sort(a,a+4);cnt=min(d,min(k/2,min(o/2,r)))*7;
		cout<<cnt+a[0]*a[0]+a[1]*a[1]+a[2]*a[2]+(a[3]+m)*(a[3]+m);
	}
	return 0;
} 


//dkkoor 2
//d r