#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  srand(time(NULL));
  ;
  long long t, k, n, m, d, u, v;
  t = 1;
  while (t--) {
    cin >> n >> m;
    vector<long long> a(n + 2);
    vector<long long> b(m + 2);
    a[0] = -1e10;
    a[n + 1] = 1e10;
    for (int i = 1; i <= n; i++) {
      cin >> a[i];
      a[i] -= i;
    }
    b[0] = 0;
    b[m + 1] = n + 1;
    for (int i = 1; i <= m; i++) {
      cin >> b[i];
    }
    bool flag = true;
    long long ans = 0;
    for (int i = 1; i <= m + 1; i++) {
      long long l, r;
      l = b[i - 1];
      r = b[i];
      if (a[r] < a[l]) {
        flag = false;
        break;
      }
      vector<long long> LIS;
      for (int j = l + 1; j < r; j++) {
        if (a[j] >= a[l] && a[j] <= a[r]) {
          auto pos = upper_bound(LIS.begin(), LIS.end(), a[j]);
          if (pos == LIS.end())
            LIS.push_back(a[j]);
          else
            *pos = a[j];
        }
      }
      ans += (r - l - 1 - LIS.size());
    }
    if (flag) {
      cout << ans << "\n";
    } else {
      cout << -1 << "\n";
    }
  }
  return 0;
}
