#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int B = 40;
const ll W = 1ll << B;
int n, m;
const int N = 10005;
vector<int> e[N], ord;
int d[N];
struct bigint {
    int len;
    ll a[1005];
} a[N];

bigint operator+(const bigint &x, const bigint &y) {
    bigint z;
    z.len = max(x.len, y.len);
    ll t = 0;
    for (int i = 0; i < z.len; i++) {
        z.a[i] = t + x.a[i] + y.a[i];
        t = (z.a[i] >> B);
        z.a[i] = (z.a[i] & (W - 1));
    }
    while (t) {
        z.a[z.len] = (t & (W - 1));
        z.len++;
        t >>= B;
    }
    return z;
}
int bl(ll x) {
    int sum = 0;
    while (x) {
        ++sum;
        x >>= 1;
    }
    return sum - 1;
}
int main() {
    scanf("%d%d", &n, &m);
    for (int i = 1, x; i <= n; ++i) {
        scanf("%d", &x);
        a[i].len = 1;
        a[i].a[0] = x;
    }
    for (int i = 1, u, v; i <= m; ++i) {
        scanf("%d%d", &u, &v);
        e[u].push_back(v);
        d[v]++;
    }
    queue<int> q;
    for (int i = 1; i <= n; ++i)
        if (d[i] == 0)
            q.push(i);
    while (!q.empty()) {
        int x = q.front();
        q.pop();
        ord.push_back(x);
        for (int y : e[x]) {
            --d[y];
            if (!d[y])
                q.push(y);
        }
    }
    for (int x : ord) {
        a[x] = a[x] + a[x];
        for (int y : e[x])
            a[y] = a[x] + a[y];
    }
    int o = ord.back();
    if (a[o].len == 1 && a[o].a[0] == 0) {
        puts("0");
        return 0;
    }
    cout << ((a[o].len - 1) * B + bl(a[o].a[a[o].len - 1])) << endl;
    return 0;
}