#include<stdio.h>

int main(void)
{
  int map[1000];
  int n, m, loc = 0, breakpoint = 0;
  int i, x;

  scanf("%d %d", &n, &m);

  for(i = 0; i < n; i++)
    {
      scanf("%d", &map[i]);
    }

  for(i = 0; i < m; i++)
    {
      scanf("%d", &x);
      if(breakpoint) continue;
      loc += x;
      if(loc >= n - 1)
	{
	  breakpoint = i + 1;
	  continue;
	}
      loc += map[loc];
    }

  printf("%d\n", breakpoint);

  return 0;
}