#include <bits/stdc++.h>
using namespace std;
int get(long long l, long long r, vector<long long> v) {
  vector<long long> dp;
  for (auto i : v) {
    if (i < l or i > r) continue;
    auto it = upper_bound(dp.begin(), dp.end(), i);
    if (it == dp.end())
      dp.push_back(i);
    else
      *it = i;
  }
  return dp.size();
}
void nikal_pehli_fursat_mai() {
  int n, k;
  cin >> n >> k;
  vector<long long> a(n + 2), b(k + 2, 0);
  a[0] = -1e18;
  a[n + 1] = 1e18;
  b[k + 1] = n + 1;
  for (int i = 1; i < n + 1; i++) {
    cin >> a[i];
    a[i] -= i;
  }
  for (int i = 1; i < k + 1; i++) {
    cin >> b[i];
  }
  long long ans = 0;
  for (int i = 0; i < k + 1; i++) {
    int l = b[i], h = b[i + 1];
    if (a[l] > a[h]) {
      cout << -1 << "\n";
      return;
    }
    if (b[i + 1] - b[i] <= 1) continue;
    ans += h - l - 1 -
           get(a[l], a[h], vector<long long>(a.begin() + l + 1, a.begin() + h));
  }
  cout << ans << "\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int tc = 1;
  while (tc--) {
    nikal_pehli_fursat_mai();
  }
}
