#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using i64 = long long;
using u64 = unsigned long long;
#define rep(i,n) for(i64 i=0; i<i64(n); i++)
const i64 INF = 1001001001001001001;
template<typename A> void chmin(A& l, const A& r){ if(r < l) l = r; }
template<typename A> void chmax(A& l, const A& r){ if(l < r) l = r; }
using namespace std;


void testcase(){
    i64 N, T; cin >> N >> T;
    vector<i64> A(N-1); rep(i,N-1) cin >> A[i];
    i64 K; cin >> K;
    vector<i64> X(N); rep(i,K){ i64 x; cin >> x; X[x-1] += 1; }
    i64 cnt = 0, tim = T;
    i64 ans = 0;
    rep(i,N-1){
        i64 w = 0;
        if(tim <= A[i]) w = (A[i] - tim) / 10 + 1;
        if(cnt < w){ cout << "-1\n"; return; }
        cnt -= w;
        ans += w;
        cnt += X[i+1];
        tim += w * 10 - A[i];
    }
    cout << ans << "\n";
}

int main(){
    ios::sync_with_stdio(false); cin.tie(nullptr);
    rep(f,1) testcase();
    return 0;
}