use std::io::Write;
use std::collections::*;

type Map<K, V> = BTreeMap<K, V>;
type Set<T> = BTreeSet<T>;
type Deque<T> = VecDeque<T>;

fn main() {
    input! {
        n: usize,
        m: usize,
        a: [u64; n],
        e: [(usize1, usize1); m],
    }
    if a.iter().all(|a| *a == 0) {
        println!("0");
        return;
    }
    let mut g = vec![vec![]; n];
    let mut h = vec![vec![]; n];
    let mut deg = vec![0; n];
    for &(s, t) in e.iter() {
        g[s].push(t);
        h[t].push(s);
        deg[s] += 1;
    }
    let mut topo = vec![];
    topo.push(g.iter().position(|g| g.is_empty()).unwrap());
    for i in 0..n {
        let v = topo[i];
        for &u in h[v].iter() {
            deg[u] -= 1;
            if deg[u] == 0 {
                topo.push(u);
            }
        }
    }
    let norm = |a: &mut Vec<u64>| {
        while a.last().map_or(false, |p| *p == 0) {
            a.pop();
        }
    };
    let add = |a: &mut Vec<u64>, b: &[u64]| {
        let len = a.len().max(b.len());
        a.resize(len + 1, 0);
        let mut carry = 0;
        for (a, b) in a.iter_mut().zip(b.iter()) {
            let (x, c) = (*a).overflowing_add(*b);
            let (x, d) = x.overflowing_add(carry);
            *a = x;
            if c || d {
                carry = 1;
            } else {
                carry = 0;
            }
        }
        norm(a);
    };
    let mut dp = a.iter().map(|a| vec![*a]).collect::<Vec<_>>();
    for &v in topo[1..].iter().rev() {
        let mut src = std::mem::take(&mut dp[v]);
        let len = src.len();
        src.push(0);
        for i in (0..len).rev() {
            let v = src[i];
            if v >> 63 & 1 == 1 {
                src[i + 1] += 1;
            }
            src[i] = v << 1;
        }
        norm(&mut src);
        for &u in g[v].iter() {
            add(&mut dp[u], &src);
        }
    }
    let dp = &mut dp[topo[0]];
    norm(dp);
    let mut x = 63;
    while *dp.last().unwrap() >> x & 1 == 0 {
        x -= 1;
    }
    let ans = (dp.len() - 1) * 64 + x + 1;
    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 ----------