#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 5;
int n, k;
int a[N], b[N], mark[N];
int mn[N], mx[N];
int get(vector<int> v) {
  multiset<int> s;
  for (auto &it : v) {
    s.insert(it);
    auto iter = s.upper_bound(it);
    if (iter != s.end()) s.erase(iter);
  }
  return s.size();
}
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cin >> n >> k;
  for (int i = 1; i <= n; i++) cin >> a[i];
  vector<int> v;
  for (int i = 1; i <= k; i++) {
    cin >> b[i];
    mark[b[i]] = 1;
    v.push_back(a[b[i]]);
  }
  for (int i = 1; i < k; i++) {
    if (v[i] <= v[i - 1]) {
      cout << -1;
      return 0;
    }
  }
  for (int i = 1; i <= k - 1; i++) {
    int diff = a[b[i + 1]] - a[b[i]];
    int mn = b[i + 1] - b[i];
    if (diff < mn) {
      cout << -1;
      return 0;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (mark[i]) mn[i] = a[i], mx[i] = a[i];
  }
  mn[0] = -1e9;
  for (int i = 1; i <= n; i++) {
    if (mn[i] == 0) mn[i] = -2e9;
    mn[i] = max(mn[i], mn[i - 1] + 1);
  }
  mx[n + 1] = 2e9;
  for (int i = n; i >= 1; i--) {
    if (mx[i] == 0) mx[i] = 2e9;
    mx[i] = min(mx[i], mx[i + 1] - 1);
  }
  v.clear();
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    if (a[i] < mn[i] || a[i] > mx[i])
      ans++;
    else if (!mark[i])
      v.push_back(a[i] - i);
  }
  ans += v.size() - get(v);
  cout << ans;
  return 0;
}
