#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

int N,M;
int A[808080],B[80800];
int vis[808080];
void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>M;
	FOR(i,2*N+1) cin>>A[i];
	queue<int> Q;
	FOR(i,M) {
		cin>>B[i];
		vis[B[i]]=1;
		Q.push(B[i]);
	}
	
	int ret=M;
	while(Q.size()) {
		x=Q.front();
		Q.pop();
		FOR(i,M) if(vis[A[x+B[i]]]==0) {
			vis[A[x+B[i]]]=1;
			ret++;
			Q.push(A[x+B[i]]);
		}
	}
	cout<<ret<<endl;
	
	
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}