#include<stdio.h>
int a[8003], s[4003];
int v[4003];
int stack[4003], ss;
int main()
{
	int n, m;
	scanf("%d %d", &n, &m);
	int i, j;
	for (i = 0; i <= 2 * n; i++)
		scanf("%d", &a[i]);
	for (i = 0; i < m; i++)
		scanf("%d", &s[i]);
	for (i = 0; i <= n; i++)
		v[i] = 0;
	ss = 0;
	for (i = 0; i < m; i++)
	{
		v[s[i]] = 1;
		stack[ss++] = s[i];
	}
	while (ss > 0)
	{
		i = stack[--ss];
		for (j = 0; j < m; j++)
		{
			if (v[a[i + s[j]]] == 0)
			{
				v[a[i + s[j]]]++;
				stack[ss++] = a[i + s[j]];
			}
		}
	}
	int ans = 0;
	for (i = 0; i <= n; i++)
		ans += v[i];
	printf("%d\n", ans);
	return 0;
}