#include <bits/stdc++.h>
using namespace std;
int n, m, a[500010], b[500010], s[500010];
int count(int l, int r) {
  int n = 0;
  for (int i = l + 1; i <= r; i++) {
    if (a[i] < a[l]) continue;
    int f = upper_bound(s, s + n, a[i]) - s;
    if (f == n) {
      s[n++] = a[i];
    } else {
      s[f] = a[i];
    }
    if (i == r) return (r - l) - (f + 1);
  }
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
    a[i] -= i;
  }
  a[0] = -1000000001;
  a[n + 1] = 1000000001;
  b[0] = 0;
  for (int i = 1; i <= m; i++) scanf("%d", &b[i]);
  b[m + 1] = n + 1;
  m++;
  int fl = 0;
  for (int i = 0; i < m; i++) {
    if (a[b[i]] > a[b[i + 1]]) {
      fl = 1;
      break;
    }
  }
  if (fl == 1) {
    printf("-1");
    return 0;
  }
  int cnt = 0;
  for (int i = 0; i < m; i++) {
    cnt += count(b[i], b[i + 1]);
  }
  printf("%d\n", cnt);
}
