#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  vector<int> arr(n + 2), b(k);
  const int oo = 1 << 30;
  for (int i = 1; i <= n; ++i) {
    cin >> arr[i];
    arr[i] -= i;
  }
  arr[0] = -oo;
  arr[n + 1] = oo;
  for (int i = 0; i < k; ++i) {
    cin >> b[i];
  }
  b.insert(b.begin(), 0);
  b.insert(b.end(), n + 1);
  n += 2;
  k += 2;
  int ans = 0;
  for (int i = 0; i < k - 1; ++i) {
    if (arr[b[i]] > arr[b[i + 1]]) {
      cout << -1 << endl;
      return 0;
    }
    vector<int> lis;
    for (int j = b[i] + 1; j < b[i + 1]; ++j) {
      if (arr[b[i]] <= arr[j] && arr[j] <= arr[b[i + 1]]) {
        auto p = upper_bound(lis.begin(), lis.end(), arr[j]);
        if (p == lis.end()) {
          lis.push_back(arr[j]);
        } else {
          *p = arr[j];
        }
      }
    }
    ans += b[i + 1] - 1 - (b[i] + 1) + 1 - (int)lis.size();
  }
  cout << ans << endl;
}
