#include<bits/stdc++.h>

#define ll long long
#define ull unsigned long long
#define fi first
#define se second
#define mkp make_pair
#define pb push_back
#define gc getchar
#define pc putchar
#define pii pair<int,int>
#define pil pair<int,long long>
#define pli pair<long long,int>
#define pll pair<long long,long long>
#define For(i,l,r) for(int i=(l);i<=(r);i++)
#define IFor(i,r,l) for(int i=(r);i>=(l);i--)
#define for_son(u) for(int e=head[u];e;e=nxt[e])
#define eps (1e-8)
#define INF 0x3f3f3f3f

using namespace std;

char aa;

namespace ljh
{

namespace IO
{
//    const int SIZ=1<<20;
//    char ibuf[SIZ],*p1,*p2,obuf[SIZ],*p3=obuf;
//  #define gc() (p1==p2&&(p2=(p1=ibuf)+fread(ibuf,1,SIZ,stdin),p1==p2)?EOF:*p1++)
//  void pc(const char &c)
//  {
//      if(p3-obuf==SIZ) fwrite(obuf,1,SIZ,stdout),p3=obuf;
//      *p3++=c;
//  }
    int rd()
    {
        int x=0,f=1;
        char ch=gc();
        while(!isdigit(ch)){
            if(ch=='-') f=-1;
            ch=gc();
        }
        while(isdigit(ch)){
            x=x*10+ch-'0';
            ch=gc();
        }
        return x*f;
    }
    void wr(int x,char ch)
    {
        if(x<0) x=-x,pc('-');
        static char st[12];
        int top=0;
        do{
            st[top++]=x%10+'0',x/=10;
        }while(x);
        while(top) pc(st[--top]);
        pc(ch);
    }
    ll ll_rd()
    {
        ll x=0;
        int f=1;
        char ch=gc();
        while(!isdigit(ch)){
            if(ch=='-') f=-1;
            ch=gc();
        }
        while(isdigit(ch)){
            x=x*10+ch-'0';
            ch=gc();
        }
        return x*f;
    }
    void ll_wr(ll x,char ch)
    {
        if(x<0) x=-x,pc('-');
        static char st[22];
        int top=0;
        do{
            st[top++]=x%10+'0',x/=10;
        }while(x);
        while(top) pc(st[--top]);
        pc(ch);
    }
}

using namespace IO;

const int N=5e4+5,mod=0;

namespace tem
{
    int qpow(int a,int b,int p)
    {
        a%=p;
        int res=1;
        while(b){
            if(b&1) res=1ll*res*a%p;
            a=1ll*a*a%p;
            b>>=1;
        }
        return res;
    }
    int get_inv(int x){return qpow(x,mod-2,mod);}
    int lowbit(int x){return x&(-x);}
    void add(int &x,const int y){ x=(x+y>=mod?x+y-mod:x+y); }
//  int get(int x) {return fa[x]==x?x:fa[x]=get(fa[x])};
    int gcd(int a,int b){return b?gcd(b,a%b):a;}
    int lcm(int a,int b){return a/gcd(a,b)*b;}
}

using namespace tem;

struct Node{
	int fa,len,nxt[2];
	#define fa(p) node[p].fa
	#define len(p) node[p].len
	#define nxt(p,c) node[p].nxt[c]
}node[N<<1];

int lst=1,idx=1,siz[N<<1];
int cnt=1,head[N<<1],nxt[N<<1],to[N<<1];

int n,k,f[N];
ll now=0;
char s[N];

void add_edge(int u,int v){nxt[++cnt]=head[u],to[cnt]=v,head[u]=cnt;}

void ext(int c)
{
	int p=lst,np=lst=++idx;
	len(np)=len(p)+1;
	for(;p&&!nxt(p,c);p=fa(p)) nxt(p,c)=np;
	if(!p) fa(np)=1;
	else{
		int q=nxt(p,c);
		if(len(q)==len(p)+1) fa(np)=q;
		else{
			int nq=++idx;
			node[nq]=node[q],len(nq)=len(p)+1;
			fa(q)=fa(np)=nq;
			for(;p&&nxt(p,c)==q;p=fa(p)) nxt(p,c)=nq;
		}
	}
}

void dfs(int u)
{
//	cerr<<u<<endl;
	for_son(u){
		int v=to[e];
		dfs(v);
		siz[u]+=siz[v];
	}
	now+=1ll*(len(u)-len(fa(u)))*f[siz[u]];
}

ll calc(int l,int r)
{
//	cerr<<"calc "<<l<<" "<<r;
	cnt=1;
	For(i,1,idx) node[i]={0,0,{0,0}},siz[i]=head[i]=0;
	now=0;
	lst=idx=1;
	For(i,l,r) ext(s[i]-'0'),siz[lst]=1;
	For(i,2,idx) add_edge(fa(i),i);
	dfs(1);
//	cerr<<" = "<<now<<endl;
	return now;
}

bool check(ll lim)
{
	int res=0;
	for(int i=1,j=0;i<=n;i=j+1){
		if(k>5){
			int stp=1;
			j=i-1;
			while(stp){
				if(j+stp<=n){
					if(calc(i,j+stp)<=lim) j+=stp,stp<<=1;
					else stp>>=1;
				} 
				else stp>>=1;
			}
		}
		else{
			int l=i,r=n;
			while(l<r){
				int mid=(l+r+1)>>1;
				if(calc(i,mid)<=lim) l=mid;
				else r=mid-1;
			}
			j=l;
		}
		if(j<i) return false;
		++res;
		if(res>k) return false;
	}
	return res<=k;
}

void main()
{
	n=rd(),k=rd();
	scanf("%s",s+1);
	For(i,1,n) f[i]=rd();
	ll l=0,r=1ll*n*f[n];
	while(l<r){
		ll mid=(l+r)>>1;
		if(check(mid)) r=mid;
		else l=mid+1;
//		cerr<<"check "<<mid<<" = "<<check(mid)<<endl;
	}
	ll_wr(r,'\n');
//  fwrite(obuf,1,p3-obuf,stdout);
}

/*
*/

}

char bb;

signed main()
{
//  freopen("1.in","r",stdin);
//  freopen("ex_separate4.out","w",stdout);    
//  int st=clock();
    ljh::main();
//  int ed=clock();
//  cerr<<ed-st<<"ms"<<endl;
//      cerr<<(&bb-&aa)/1024/1024<<"MB"<<endl;
//  fclose(stdin);
//  fclose(stdout);
    return 0;
}