#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int NAX = 2e5 + 5, MOD = 1000000007;
struct BIT {
  int N;
  vector<long long> bit;
  void init(int n) {
    N = n;
    bit.assign(n + 1, 0);
  }
  void updateMax(int idx, long long val) {
    while (idx <= N) {
      bit[idx] = max(bit[idx], val);
      idx = idx | (1 + idx);
    }
  }
  long long prefMax(int idx) {
    long long ans = -2e9;
    while (idx > 0) {
      ans = max(ans, bit[idx]);
      idx = (idx & (idx + 1)) - 1;
    }
    return ans;
  }
};
int LIS(vector<int> a) {
  int n = a.size();
  if (n == 0) return 0;
  ;
  auto aCopy = a;
  sort(aCopy.begin(), aCopy.end());
  aCopy.resize(unique(aCopy.begin(), aCopy.end()) - aCopy.begin());
  for (auto &x : a)
    x = lower_bound(aCopy.begin(), aCopy.end(), x) - aCopy.begin();
  BIT b;
  b.init(n + 2);
  for (size_t i = 0; i < n; i++) {
    auto prev = b.prefMax(a[i] + 1);
    if (prev < 0) prev = 0;
    b.updateMax(a[i] + 1, prev + 1);
  }
  auto res = b.prefMax(n + 1);
  ;
  return res;
}
void solveCase() {
  int n, k;
  cin >> n >> k;
  vector<ll> a(n + 2);
  vector<bool> Fixed(n + 2);
  for (size_t i = 1; i <= n; i++) cin >> a[i];
  vector<int> events;
  events.push_back(0);
  for (size_t i = 0; i < k; i++) {
    int x;
    cin >> x;
    events.push_back(x);
    Fixed[x] = 1;
  }
  Fixed[0] = Fixed[n + 1] = 1;
  events.push_back(n + 1);
  a[0] = -MOD, a[n + 1] = 2 * MOD;
  int res = 0;
  for (size_t i = 0; i + 1 < events.size(); i++) {
    int lptr = events[i], rptr = events[i + 1];
    if (a[rptr] - a[lptr] >= rptr - lptr) {
      vector<int> myList;
      int ptr = lptr + 1;
      while (ptr < rptr) {
        if ((a[lptr] - lptr) <= (a[ptr] - ptr) &&
            (a[ptr] - ptr) <= (a[rptr] - rptr))
          myList.push_back(a[ptr] - ptr);
        else
          res++;
        ++ptr;
      }
      res += myList.size() - LIS(myList);
    } else {
      cout << -1 << '\n';
      return;
    }
  }
  cout << res << '\n';
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int t = 1;
  for (int i = 1; i <= t; ++i) {
    solveCase();
  }
  return 0;
}
