#include<bits/stdc++.h>
using namespace std;
#define rep(i,x,y) for (int i=(x);i<=(y);i++)
#define drep(i,y,x) for (int i=(y);i>=(x);i--)
#define pii pair<int,int>
#define fir first
#define sec second
#define MP make_pair
template<typename T> bool chkmin(T &x,T y){return x>y?x=y,1:0;}
template<typename T> bool chkmax(T &x,T y){return x<y?x=y,1:0;}
void file() {
    #ifdef zqj
    freopen("a.in","r",stdin);
    #endif
}
#define sz 202020

int n,m;
int a[sz],b[sz];

int check(int t) {
    static int _a[sz];
    rep(i,1,n) _a[i]=a[i];
    rep(i,1,min(t,m)) _a[b[i]]^=1;
    int cc=0;
    rep(i,1,n) cc+=_a[i];
    return cc<=t;
}

int main() {
    file();
    ios::sync_with_stdio(false),cin.tie(0);
    cin>>n>>m;
    rep(i,1,n) cin>>a[i];
    rep(i,1,m) cin>>b[i];
    int l=0,r=n+1,ans=-1;
    while (l<=r) {
        int mid=(l+r)>>1;
        if (check(mid)) ans=mid,r=mid-1;
        else l=mid+1;
    }
    cout<<ans<<'\n';
    return 0;
}