#include <bits/stdc++.h>
using namespace std;
const int maxn = 6e5 + 5;
const int MAX = 0x3f3f3f3f;
int dp[maxn], vis[maxn], a[maxn], b[maxn];
int solve(int x) {
  int r = b[x], l = b[x - 1];
  if (a[r] < a[l]) return -1;
  int cnt = 0;
  dp[0] = a[l];
  for (int i = l + 1; i < r; i++) {
    if (a[i] >= dp[cnt]) {
      dp[++cnt] = a[i];
    } else {
      int id = upper_bound(dp, dp + cnt + 1, a[i]) - dp;
      if (id == 0) {
        continue;
      } else {
        dp[id] = a[i];
      }
    }
  }
  int id = upper_bound(dp, dp + cnt + 1, a[r]) - dp;
  int res = id - 1;
  return r - l - 1 - res;
}
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
    a[i] = a[i] - i;
  }
  for (int i = 1; i <= k; i++) {
    scanf("%d", &b[i]);
  }
  a[0] = -MAX;
  b[0] = 0;
  a[n + 1] = MAX;
  b[k + 1] = n + 1;
  int ans = 0;
  for (int i = 1; i <= k + 1; i++) {
    int tmp = solve(i);
    if (tmp == -1) {
      printf("-1\n");
      return 0;
    } else {
      ans += tmp;
    }
  }
  printf("%d\n", ans);
}
