#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 10;
const int INF = 0x3f3f3f3f;
int a[N], b[N], f[N], in_b[N], tot, n;
int cal() {
  tot = 0;
  f[++tot] = a[1];
  int las = in_b[1] ? 1 : 0;
  for (int i = 2; i <= n; ++i) {
    if (a[i] >= f[tot]) {
      f[++tot] = a[i];
      if (in_b[i]) las = tot;
    } else {
      int pos = upper_bound(f + 1, f + 1 + tot, a[i]) - f;
      if (pos <= las) continue;
      f[pos] = a[i];
      if (in_b[i]) las = pos, tot = pos;
    }
  }
  return tot;
}
int main() {
  int k;
  cin >> n >> k;
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &a[i]);
    a[i] -= i;
  }
  for (int i = 1; i <= k; ++i) {
    scanf("%d", &b[i]);
    in_b[b[i]] = 1;
  }
  b[k + 1] = n + 1;
  a[0] = -INF, a[n + 1] = INF;
  for (int i = 1; i <= k; ++i) {
    if (a[b[i]] < a[b[i - 1]]) {
      puts("-1");
      return 0;
    }
  }
  printf("%d\n", n - cal());
  return 0;
}
