#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 10;
int a[N], b[N];
int n, k;
int dp[N];
int solve(int l, int r) {
  int tot = 0;
  for (int i = l + 1; i < r; i++) {
    if (a[i] >= a[l] && a[i] <= a[r]) {
      if (tot == 0 || a[i] >= dp[tot]) {
        dp[++tot] = a[i];
      } else {
        int p = upper_bound(dp + 1, dp + tot + 1, a[i]) - dp;
        dp[p] = a[i];
      }
    }
  }
  return r - l - 1 - tot;
}
int main() {
  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]);
  }
  a[0] = -(1 << 30);
  a[n + 1] = 1 << 30;
  for (int i = 0; i <= n + 1; i++) {
    a[i] -= i;
  }
  b[k + 1] = n + 1;
  int ans = 0;
  for (int i = 0; i <= k; i++) {
    if (a[b[i + 1]] < a[b[i]]) {
      printf("-1\n");
      return 0;
    }
    ans += solve(b[i], b[i + 1]);
  }
  printf("%d\n", ans);
  return 0;
}
