#include <bits/stdc++.h>
using namespace std;
int LIS(vector<int> a) {
  int n = (int)a.size() - 1;
  vector<int> s(n + 1);
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    int pos = upper_bound(s.begin() + 1, s.begin() + ans + 1, a[i]) - s.begin();
    s[pos] = a[i];
    ans = max(ans, pos);
  }
  return ans;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, k;
  cin >> n >> k;
  vector<int> a(n + 2);
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    a[i] -= i;
  }
  a[0] = INT_MIN / 2;
  a[n + 1] = INT_MAX / 2;
  vector<int> b(k + 2);
  bool ok = true;
  for (int i = 1; i <= k; i++) {
    cin >> b[i];
    if (i > 1) {
      if (a[b[i - 1]] > a[b[i]]) {
        ok = false;
      }
    }
  }
  b[k + 1] = n + 1;
  if (!ok) {
    cout << "-1\n";
    return 0;
  }
  int ans = n - k;
  for (int i = 1; i <= k + 1; i++) {
    vector<int> tmp{0};
    for (int j = b[i - 1] + 1; j <= b[i] - 1; j++) {
      if (a[j] >= a[b[i - 1]] && a[j] <= a[b[i]]) {
        tmp.push_back(a[j]);
      }
    }
    int keep = LIS(tmp);
    ans -= keep;
  }
  cout << ans << "\n";
  return 0;
}
