//Twenty

#include<algorithm>

#include<iostream>

#include<cstring>

#include<cstdlib>

#include<cstdio>

#include<vector>

#include<cmath>

#include<queue>

#include<ctime>

const int maxn=2005;

typedef long long LL;

using namespace std;

int n,m,cost[30],a,b;

char s[maxn];

namespace fastIO {

    const int sz=1<<15|1;

    char ch,buf[sz],*l,*r;

    void gechar(char &c) {

	    if(l==r) r=(l=buf)+fread(buf,1,sz,stdin);

	    c = l==r?(char)EOF:*l++;

	}

	template<typename T> void read(T &x) {

	    int f=1; x=0; gechar(ch);

	    while(ch!='-'&&(ch<'0'||ch>'9')) gechar(ch);

	    if(ch=='-') f=-1,gechar(ch);

	    for(;ch>='0'&&ch<='9';gechar(ch)) x=x*10+ch-'0'; x*=f;

	}

}

int dp[maxn][maxn];

void work() {

	memset(dp,127,sizeof(dp));

	for(int i=1;i<=m;i++) {

		dp[i][i]=0;

		if(s[i]==s[i+1]) dp[i][i+1]=0;

	}

	for(int l=2;l<=m;l++) {

		for(int i=1;i+l-1<=m;i++) {

			int j=i+l-1;

			if(l>2&&s[i]==s[j]) dp[i][j]=min(dp[i][j],dp[i+1][j-1]);

			dp[i][j]=min(dp[i][j],dp[i][j-1]+cost[s[j]-'a']);

			dp[i][j]=min(dp[i][j],dp[i+1][j]+cost[s[i]-'a']);

		}

	}

	printf("%d\n",dp[1][m]);

}

void init() {

	fastIO::read(n);

	fastIO::read(m);

	char ch;

	fastIO::gechar(ch);

	while(ch<'a'||ch>'z')

		fastIO::gechar(ch);

	for(int i=1;i<=m;i++) {

		s[i]=ch; fastIO::gechar(ch);

    }

    for(int i=1;i<=n;i++) {

		fastIO::gechar(ch);

		while(ch<'a'||ch>'z') fastIO::gechar(ch);

		fastIO::read(a);

		fastIO::read(b);

		cost[ch-'a']=min(a,b);

	}

}

//#define DEBUG

int main()  {

#ifdef DEBUG

    freopen("1.in","r",stdin);

    //freopen(".out","w",stdout);

#endif

    init();

    work();

    return 0;

}