#include <bits/stdc++.h>  
 
using namespace std;
 
typedef long long ll;
typedef long double ld;
typedef pair<int,int> p32;
typedef pair<ll,ll> p64;
typedef pair<double,double> pdd;
typedef vector<ll> v64;
typedef vector<int> v32;
typedef vector<vector<int> > vv32;
typedef vector<vector<ll> > vv64;
typedef vector<vector<p64> > vvp64;
typedef vector<p64> vp64;
typedef vector<p32> vp32;
ll MOD = 998244353;
double eps = 1e-12;
#define forn(i,e) for(ll i = 0; i < e; i++)
#define forsn(i,s,e) for(ll i = s; i < e; i++)
#define rforn(i,s) for(ll i = s; i >= 0; i--)
#define rforsn(i,s,e) for(ll i = s; i >= e; i--)
#define ln "\n"
#define dbg(x) cout<<#x<<" = "<<x<<ln
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define INF 2e18
#define all(x) (x).begin(), (x).end()
#define sz(x) ((ll)(x).size())
 
int main()
{
   // ifstream cin("in.txt");
    int n;
    cin >> n;
    int m;
    cin >> m;

    vector<int> workHours(n + 1);
    for(int i = 1; i <= n; i++){
        cin >> workHours[i];
    }

    vector<int> deadlines(m + 1);
    for(int i = 1; i <= m; i++){
        cin >> deadlines[i];
    }

    sort(deadlines.begin() + 1, deadlines.end());

    int ans = 0;

    for(int i = 1; i <= m; i++){
        if(deadlines[i] != deadlines[i - 1]){
            workHours[deadlines[i]] += workHours[deadlines[i - 1]];
        }

        if(workHours[deadlines[i]] > 0){
            workHours[deadlines[i]]--;
            ans++;
        }
    }

    cout << ans;
    
}

/*
    2 0 6
    2 3 1 2 3



*/