#include <bits/stdc++.h>
using namespace std;
int a[500005], b[500005], ban[500050];
int dp[500005];
int main() {
  memset(dp, 0x3f, sizeof(dp));
  int n, k;
  scanf("%d %d", &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
  }
  for (int i = 1; i <= k; i++) {
    scanf("%d", &b[i]);
    ban[b[i]] = 1;
  }
  for (int i = 1; i < k; i++) {
    if (a[b[i + 1]] - a[b[i]] < b[i + 1] - b[i]) {
      puts("-1");
      return 0;
    }
  }
  for (int i = 1; i <= n; i++) {
    a[i] -= i;
  }
  int cnt = 0, lst = 0;
  for (int i = 1; i <= n; i++) {
    if (cnt == 0 || a[i] >= dp[cnt]) {
      cnt++;
      dp[cnt] = a[i];
      if (ban[i]) {
        lst = cnt;
      }
    } else {
      int p = upper_bound(dp + 1, dp + 1 + cnt, a[i]) - dp;
      if (p <= lst) continue;
      dp[p] = a[i];
      if (ban[i]) {
        lst = p;
        cnt = p;
      }
    }
  }
  printf("%d\n", n - cnt);
}
