//writer:Oier_szc

#include <bits/stdc++.h>
//#include <windows.h>
#define ED cerr<<endl;
#define TS cerr<<"I AK IOI"<<endl;
#define cr(x) cerr<<x<<endl;
#define cr2(x,y) cerr<<x<<" "<<y<<endl;
#define cr3(x,y,z) cerr<<x<<" "<<y<<" "<<z<<endl;
#define cr4(x,y,z,w) cerr<<x<<" "<<y<<" "<<z<<" "<<w<<endl;
#define pii pair<int,int>
#define fi first
#define se second
#define ll long long
//#define ull unsigned long long
using namespace std;
const int N=1e6+5,mod=1e9+7;
const ll INF=1e16;
int n,c;
int v[30];
char _s[N],s[N<<1];
int p[N<<1],len=0;
ll dp[N<<1];
int q[N<<1],l=1,r=0;

void init() {
	s[++len]='$',s[++len]='#';
	for(int i=1;i<=n;++i) s[++len]=_s[i],s[++len]='#';
	s[++len]='^';
}

void manacher() {
	int mr=0,mid=0;
	for(int i=1;i<len;++i) {
		if(i<mr) p[i]=min(p[2*mid-i],mr-i);
		else p[i]=1;
		while(s[i+p[i]]==s[i-p[i]]) ++p[i];
		if(i+p[i]>mr) mr=i+p[i],mid=i;
	}
}


int main()
{
	scanf("%d%d",&n,&c);
	for(int i=0;i<26;++i) {
		scanf("%d",&v[i]);
	}
	scanf("%s",_s+1);
	init(),manacher();
	for(int i=1;i<=len;++i) dp[i]=INF;
	dp[1]=0;
	int fr;
	for(int i=3;i<=len-2;++i) {
		while(l<=r&&(q[l]+p[q[l]]-1<i||2*q[l]-i<3)) ++l;
		if(i&1) {
			dp[i]=dp[i-2]+v[s[i]-'a'];
			if(l<=r) {
				fr=q[l];
				if(!(fr&1)) --fr;
				dp[i]=min(dp[i],dp[fr]+c);
			}
		}
		q[++r]=i;
	}
	printf("%lld\n",dp[len-2]);
	return 0;
}