use proconio::{input, marker::Usize1};

fn main() {
    match solve() {
        Some(ans) => println!("{}", ans),
        None => println!("-1"),
    }
}

fn solve() -> Option<usize> {
    input! {
        (n, t): (usize, usize),
        tt: [usize; n - 1],
        k: usize,
        xx: [Usize1; k],
    }

    let mut exists_clock_flags = vec![false; n];
    for &x in &xx {
        exists_clock_flags[x] = true;
    }

    let mut count = t;
    let mut rem_clocks = 0_usize;
    let mut used_clocks = 0_usize;
    for (pos, &t) in tt.iter().enumerate() {
        rem_clocks += exists_clock_flags[pos] as usize;

        let require_clocks = (t + 10).saturating_sub(count) / 10;

        if rem_clocks < require_clocks {
            return None;
        }

        rem_clocks -= require_clocks;
        used_clocks += require_clocks;

        count = count + 10 * require_clocks - t;
    }

    Some(used_clocks)
}