#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll MAXN=5e6+5;
char s[MAXN];
ll p, mana[MAXN], sz, mid, r, n, cost, c[30], dp[MAXN];
deque<pair<ll, ll> > q;
inline void read(){
	char c=getchar();
	s[0]='?';
	s[++sz]='!';
	while(c>'z'||c<'a'){
		c=getchar();
	}
	while(c>='a'&&c<='z'){
		s[++sz]=c;
		s[++sz]='!';
		c=getchar();
	}
}
int main(){
	cin>>n>>cost;
	for(ll i=0; i<26; i++){
		cin>>c[i];
	}
	read();
	for(ll i=1; i<=sz; i++){
		if(i<=r){
			mana[i]=min(mana[mid*2-i], r-i+1);
		}
		while(s[i-mana[i]]==s[i+mana[i]]){
			mana[i]++;
		}
		if(i+mana[i]>r){
			r=i+mana[i]-1;
			mid=i;
		}
	}
	memset(dp, 0x3f, sizeof(dp));
	dp[0]=0;
	for(ll i=2; i<=sz; i+=2){
		dp[i]=dp[i-2]+c[s[i]-'a'];
		while(q.size()&&q.front().second<i){
			q.pop_front();
		}
		if(q.size()){
			dp[i]=min(dp[i], q.front().first+cost);
		}
		q.push_back({dp[i], i+max(mana[i]-1, mana[i+1])});
	}
	cout<<dp[sz-1];
    return 0;
}