#include<bits/stdc++.h>
using namespace std;
#define N 2000010
#define int long long
#define db long double
#define pii pair<int,int>
#define st first
#define ed second
#define mkp make_pair
#define pb push_back
#define eps 1e-9
#define mod 998244353
#define mod2 1000000007
#define bs 13131
#define bs2 131
#define INF 0x3f3f3f3f3f3f3f3f
#define il inline
#define vi vector<int>
#define ins insert
#define umap unordered_map
#define uset unordered_set
#define R(x) x.begin(),x.end()
#define B(x) x.begin()
#define E(x) x.end()
#define lb lower_bound
#define ub upper_bound
#define vi vector<int>
il int read(){
	int x=0,f=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-')f=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9'){
		x=(x<<1)+(x<<3)+(ch^48);
		ch=getchar();
	}
	return x*f;
}
int T=1,n,m,k;
int s[N];
int f[N],l[N];
char c[N];
int q[N],h,t=-1;
signed main(){
	n=read(),k=read();
	for(int i=1;i<=26;i++)
		scanf("%lld",&s[i]);
	scanf("%s",c+1);
	for(int i=n;i>=1;i--)
		c[i<<1]=c[i],c[i<<1|1]='$';
	c[1]='$',n=n<<1|1;
	for(int i=1;i<=n;i++)f[i]=INF;
	for(int i=1,p=0;i<=n;i++){
		if(i<p+l[p])l[i]=min(l[(p<<1)-i],p+l[p]-i);
		while(i+l[i]<=n&&i-l[i]>=1&&c[i+l[i]]==c[i-l[i]])l[i]++;
		if(p+l[p]<i+l[i])p=i;
		if(p==i){
			while(h<=t&&q[t]-l[q[t]]>p-l[p])t--;
			q[++t]=p;
		}
		if(i&1)f[i]=f[i-1];
		else {
			f[i]=f[i-1]+s[c[i]-'a'+1];
			while(h<=t&&q[h]+l[q[h]]<=i)h++;
			if(h<=t)f[i]=min(f[i],f[q[h]]+k);
		}
//		printf("%lld ",f[i]);
	}
	printf("%lld\n",f[n]);
	return 0;
}