use std::collections::*;

type Map<K, V> = BTreeMap<K, V>;

fn main() {
    input! {
        n: usize,
        m: usize,
        e: [(usize1, usize1); m],
        c: [u8; n],
    }
    let mut g = vec![0usize; n];
    for (a, b) in e {
        g[a] |= 1 << b;
        g[b] |= 1 << a;
    }
    let mut state = 0;
    for (i, c) in c.iter().enumerate() {
        state |= (*c as usize) << i;
    }
    let mut dp = Map::new();
    dp.insert(state, 0);
    let mut used = vec![false; n];
    let inf = m as i32 + 1;
    while let Some(v) = (0..n).filter(|&v| !used[v]).min_by_key(|&u| g[u].count_ones()) {
        used[v] = true;
        for u in 0..n {
            if g[v] >> u & 1 == 1 {
                let mut next = dp.clone();
                for (state, w) in dp {
                    next.entry(state ^ (1 << v) ^ (1 << u)).or_insert(inf).chmin(w + 1);
                }
                dp = next;
            }
        }
        dp = dp.into_iter().filter(|p| p.0 >> v & 1 == 0).collect();
        for g in g.iter_mut() {
            *g &= !(1 << v);
        }
    }
    let ans = dp.get(&0).map_or(-1, |p| *p);
    println!("{}", ans);
}

// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[macro_export]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[macro_export]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[macro_export]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------
// ---------- begin chmin, chmax ----------
pub trait ChangeMinMax {
    fn chmin(&mut self, x: Self) -> bool;
    fn chmax(&mut self, x: Self) -> bool;
}

impl<T: PartialOrd> ChangeMinMax for T {
    fn chmin(&mut self, x: Self) -> bool {
        *self > x && {
            *self = x;
            true
        }
    }
    fn chmax(&mut self, x: Self) -> bool {
        *self < x && {
            *self = x;
            true
        }
    }
}
// ---------- end chmin, chmax ----------