//#pragma GCC optimize("Ofast")
//#pragma GCC target("avx,avx2,fma")

#include <bits/stdc++.h>
using namespace std;
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
void debug3(int *t,int st,int n) {cerr << '{' << t[st];for(int i=st+1;i<=n;i++) cerr<<",",__print(t[i]);cerr <<"}"<<'\n';}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#define debug2(x...) cerr<<"[" << #x << "] = "; debug3(x)
#endif

#define N 500005
#define ll long long
#define ull unsigned long long
#define pii pair<int,int>
#define sf second.first
#define ss second.second
#define INF 1e9

int v[N],b[N],dp[N];
vector<int> s;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    //freopen("test.in", "r", stdin);

    int n,m,i,sol=0,l,r,j;

    cin >> n >> m ;
    for ( i = 1 ; i <= n ; i++ )
        cin >> v[i] ;
    for ( i = 1 ; i <= m ; i++ )
        cin >> b[i] ;
    v[0] = -INF ;
    v[n+1] = INF ;
    b[m+1] = n+1;
    for ( i = 1 ; i < m ; i++ )
    {
        if ( v[b[i+1]]-v[b[i]] < b[i+1]-b[i] )
        {
            cout << "-1" ;
            return 0 ;
        }
    }
    for ( i = 1 ; i <= n ; i++ )
    {
        v[i] = v[i]-i;
    }
    //debug2(v,1,n);

    for ( i = 0 ; i <= m ; i++ )
    {
        l = b[i] ;
        r = b[i+1] ;
        s.clear() ;
        for ( j = l+1 ; j < r ; j++ )
        {
            if ( v[l] <= v[j] && v[j] <= v[r] )
            {
                auto x = upper_bound(s.begin(),s.end(),v[j]) ;
                if ( x==s.end() )
                    s.push_back(v[j]) ;
                else
                    *x = v[j] ;
            }
        }
        //debug(s);
        sol += r-l-1 - s.size() ;
    }
    cout << sol ;
}