#include <bits/stdc++.h>
using namespace std;

namespace NTT{
	const int MOD=998244353,g=3,ig=332748118;
	const int MAXN=1077777;
	inline long long poww(long long x,long long y){
		long long ret=1;
		while (y){
			if (y&1) ret=ret*x,ret%=MOD;
			x*=x,x%=MOD;
			y>>=1;
		}
		return ret;
	}
	inline long long inv(long long x){return poww(x,MOD-2);}
	int R[MAXN];
	inline void fft(vector<long long> &a,int L,int ty){
		for (int i=0;i<L;i++) i>R[i]?swap(a[i],a[R[i]]):void(0);
		for (int i=1;i<L;i<<=1){
			long long w=poww(ty,(MOD-1)/(i<<1));
			for (int j=0;j<L;j+=(i<<1)){
				long long wn=1;
				for (int k=j;k<i+j;k++){
					long long t=wn*a[i+k]%MOD;
					a[i+k]=a[k]-t;a[i+k]<0?a[i+k]+=MOD:0;
					a[k]+=t;a[k]>=MOD?a[k]-=MOD:0;
					wn=wn*w%MOD;
				}
			}
		}
	}
	inline vector<long long> mul(vector<long long> A,vector<long long> B){
		int L=1,n=(int)A.size()-1,m=(int)B.size()-1;
		while (L<=n+m) L<<=1;
		A.resize(L);
		B.resize(L);
		for (int i=1;i<L;i<<=1)
			for (int j=0;j<i;j++)
				R[i+j]=R[j]+L/(i<<1);
		fft(A,L,g);
		fft(B,L,g);
		vector<long long> ret(L);
		for (int i=0;i<L;i++)
			ret[i]=A[i]*B[i]%MOD;
		fft(ret,L,ig);
		long long invL=inv(L);
		for (int i=0;i<L;i++) ret[i]=ret[i]*invL%MOD;
		ret.resize(n+m+1);
		return ret;
	}
}
namespace NTT2{
	const int MOD=1004535809,g=3;
	const int MAXN=1077777;
	inline long long poww(long long x,long long y){
		long long ret=1;
		while (y){
			if (y&1) ret=ret*x,ret%=MOD;
			x*=x,x%=MOD;
			y>>=1;
		}
		return ret;
	}
	inline long long inv(long long x){return poww(x,MOD-2);}

	const int ig=inv(g);

	int R[MAXN];
	inline void fft(vector<long long> &a,int L,int ty){
		for (int i=0;i<L;i++) i>R[i]?swap(a[i],a[R[i]]):void(0);
		for (int i=1;i<L;i<<=1){
			long long w=poww(ty,(MOD-1)/(i<<1));
			for (int j=0;j<L;j+=(i<<1)){
				long long wn=1;
				for (int k=j;k<i+j;k++){
					long long t=wn*a[i+k]%MOD;
					a[i+k]=a[k]-t;a[i+k]<0?a[i+k]+=MOD:0;
					a[k]+=t;a[k]>=MOD?a[k]-=MOD:0;
					wn=wn*w%MOD;
				}
			}
		}
	}
	inline vector<long long> mul(vector<long long> A,vector<long long> B){
		int L=1,n=(int)A.size()-1,m=(int)B.size()-1;
		while (L<=n+m) L<<=1;
		A.resize(L);
		B.resize(L);
		for (int i=1;i<L;i<<=1)
			for (int j=0;j<i;j++)
				R[i+j]=R[j]+L/(i<<1);
		fft(A,L,g);
		fft(B,L,g);
		vector<long long> ret(L);
		for (int i=0;i<L;i++)
			ret[i]=A[i]*B[i]%MOD;
		fft(ret,L,ig);
		long long invL=inv(L);
		for (int i=0;i<L;i++) ret[i]=ret[i]*invL%MOD;
		ret.resize(n+m+1);
		return ret;
	}
}

typedef long long ll;

inline long long poww(long long x,long long y,long long mod){
	long long ret=1;
	while (y){
		if (y&1) ret=ret*x,ret%=mod;
		x*=x,x%=mod;
		y>>=1;
	}
	return ret;
}
inline long long inv(long long x,long long mod){return poww(x,mod-2,mod);}

const int maxn=200000;
const ll mod=998244353;
const ll mod2=1004535809;
const ll base=114;
const ll base2=514;

ll pw[maxn+50];
ll pw2[maxn+50];
void prepare(){
	pw[0]=1;
	for (int i=1;i<=maxn;i++) pw[i]=pw[i-1]*base%mod;
	pw2[0]=1;
	for (int i=1;i<=maxn;i++) pw2[i]=pw2[i-1]*base2%mod2;
}

ll n,m;
ll p[maxn+50];

string s,t;
ll ht[maxn+50];
ll ht2[maxn+50];

int main(){
    ios_base::sync_with_stdio(false);
	prepare();

    cin >> n >> m;
	for (int i=0;i<m;i++){
		cin >> p[i];
		p[i]--;
	}
	cin >> s;
	for (int i=0;i<n;i++){
		ht[i]=(s[i]-'a'+1)*pw[i]%mod;
		if (i>0) ht[i]=(ht[i]+ht[i-1])%mod;

		ht2[i]=(s[i]-'a'+1)*pw2[i]%mod2;
		if (i>0) ht2[i]=(ht2[i]+ht2[i-1])%mod2;
	}
	map<pair<ll,ll>,ll> dict;
	for (int i=0;i+m-1<n;i++){
		pair<ll,ll> ttt;
		{
			ll ed=ht[i+m-1];
			ll st=0;
			if (i>0) st=ht[i-1];
			ll tmp=(ed-st+mod)*inv(pw[i],mod)%mod;
			ttt.first=tmp;
		}
		{
			ll ed=ht2[i+m-1];
			ll st=0;
			if (i>0) st=ht2[i-1];
			ll tmp=(ed-st+mod2)*inv(pw2[i],mod2)%mod2;
			ttt.second=tmp;
		}
		dict[ttt]++;
	}

	vector<int> order;
	{
		ll x=0;
		order.emplace_back(x);
		for (x=p[x];x!=0;x=p[x]){
			order.emplace_back(x);
		}
	}

	cin >> t;

	vector<ll> A(m);
	vector<ll> B(m+m);
	vector<ll> B2(m+m);
	for (int i=0;i<m;i++){
		A[i]=t[order[i]]-'a'+1;
		B[m-1-i+m]=B[m-1-i]=pw[order[i]];
		B2[m-1-i+m]=B2[m-1-i]=pw2[order[i]];
	}

	vector<ll> C=NTT::mul(A,B);
	vector<ll> C2=NTT2::mul(A,B2);

	int pos=-1;
	for (int i=0;i<m;i++) if (dict.count(make_pair(C[m+m-1-i],C2[m+m-1-i]))){pos=i;break;}
	cout << pos << "\n";
    return 0;
}