#include <bits/stdc++.h>


#define ll long long
#define flt double
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()


using namespace std;

const ll inf = 1e18;
mt19937 rnd(234);

const int maxn = 1e4;
const int maxln = 2e3;


ll base = 60;
ll mod = (1ll << base);
ll mask = mod - 1;

ll ln[maxn];
ll a[maxn][maxln];


void normalize_len(int i) {
    while (ln[i] > 0 and a[i][ln[i] - 1] == 0) {
        ln[i] -= 1;
    }
}


void normalize(int i) {
    ;
    ln[i] += 6;
    for (int j = 0; j < ln[i]; j += 1) {
        a[i][j + 1] += (a[i][j] >> base);
        a[i][j] &= mask;
    }
    normalize_len(i);
}


void add(int i, int j) {
    ln[j] = max(ln[i], ln[j]);
    for (int k = 0; k < ln[j]; k += 1) {
        a[j][k] += a[i][k];
    }
}

ll logarithm(int i) {
    normalize_len(i);
    if (ln[i] == 0) {
        return 0;
    }
    return base * (ln[i] - 1) + __lg(a[i][ln[i] - 1]);
}

int n, m;
vector<ll> x;
vector<vector<int>> g;
vector<int> usd;
vector<int> topsort, pos;


void dfs(int v) {
    usd[v] = true;
    for (auto to : g[v]) {
        if (usd[to]) {
            continue;
        }
        dfs(to);
    }
    pos[v] = n - (int)topsort.size() - 1;
    topsort.push_back(v);
}


int32_t main() {
    if (1) {
        ios::sync_with_stdio(0);
        cin.tie(0);
        cout.tie(0);
    }
    cin >> n >> m;
    x.resize(n);
    for (int i = 0; i < n; i += 1) {
        cin >> x[i];
    }
    g.assign(n, vector<int>());
    for (int i = 0; i < m; i += 1) {
        int u, v;;
        cin >> u >> v;
        --u;
        --v;
        g[u].push_back(v);
    }
    topsort.clear();
    usd.assign(n, false);
    pos.resize(n);
    for (int v = 0; v < n; v += 1) {
        if (usd[v]) {
            continue;
        }
        dfs(v);
    }
    reverse(all(topsort));
    for (int i = 0; i < n; i += 1) {
        int v = topsort[i];
        a[i][0] += x[v];
        ln[i] = max(ln[i], 1ll);
        add(i, i);
        normalize(i);
        int cnt = 0;
        for (auto to : g[v]) {
            int j = pos[to];
            assert(i < j);
            add(i, j);
            cnt += 1;
            if (cnt >= 8) {
                normalize(j);
                cnt = 0;
            }
        }
        normalize(j);
    }
    cout << logarithm(n - 1) << "\n";
    return 0;
}

/*
3 2
1 1 1
1 2
2 3


6 8
1 1 4 5 1 4
1 4
1 5
2 3
2 5
3 4
4 5
4 6
5 6


5 6
7 2 3 6 6
1 2
1 4
2 3
3 4
3 5
4 5

*/

answer.code: In function ‘int32_t main()’:
answer.code:126:19: error: ‘j’ was not declared in this scope
  126 |         normalize(j);
      |                   ^