#include <bits/stdc++.h>
using namespace std;
int n, k;
const int maxN = 5e5 + 10;
int a[maxN];
int b[maxN];
int d[maxN];
int solve(int l, int r) {
  if (a[l] > a[r]) {
    cout << -1;
    exit(0);
  }
  int ans = 0;
  int len = r - l + 1;
  d[0] = -1e9 - 228;
  for (int i = 1; i <= len + 1; i++) {
    d[i] = 1e9 + 228;
  }
  for (int j = l + 1; j <= r - 1; j++) {
    if (a[j] < a[l] || a[j] > a[r]) {
      ans++;
    } else {
      ans++;
      int pos = upper_bound(d, d + len + 1, a[j]) - d;
      d[pos] = a[j];
    }
  }
  for (int i = 0; i <= len + 1; i++) {
    if (d[i] > 1e9 + 10) return ans - i + 1;
  }
  assert(false);
  return -228;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    a[i] -= i;
  }
  int ans = 0;
  for (int i = 1; i <= k; i++) {
    cin >> b[i];
  }
  a[0] = -1e9;
  a[n + 1] = 1e9 + 10;
  b[0] = 0;
  b[k + 1] = n + 1;
  for (int i = 0; i <= k; i++) {
    ans += solve(b[i], b[i + 1]);
  }
  cout << ans;
  return 0;
}
