#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6 + 10;
const int mod = 998244353;
const int inf = 0x3f3f3f3f;
int a[maxn], f[maxn], b[maxn];
int solve(int l, int r) {
  int len = 1, res = 1;
  f[len] = a[l];
  for (int i = l + 1; i <= r; i++) {
    if (a[i] >= f[len]) {
      f[++len] = a[i];
    } else {
      int tmp = upper_bound(f + 1, f + len + 1, a[i]) - f;
      if (tmp != 1) f[tmp] = a[i];
    }
  }
  res = upper_bound(f + 1, f + len + 1, a[r]) - f - 1;
  return (r + 1 - l) - res;
}
int main() {
  int n, k;
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    a[i] -= i;
  }
  for (int i = 1; i <= k; i++) cin >> b[i];
  for (int i = 1; i < k; i++) {
    if (a[b[i]] > a[b[i + 1]]) {
      puts("-1");
      return 0;
    }
  }
  b[0] = 0, b[k + 1] = n + 1;
  a[0] = -inf, a[n + 1] = inf;
  int ans = 0;
  for (int i = 1; i <= k + 1; i++) {
    ans += solve(b[i - 1], b[i]);
  }
  cout << ans << endl;
  return 0;
}
