#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
set<pair<int, int> > topo[2];
vector<int> G[maxn], R[maxn];
int n, m, code[maxn], in[maxn], out[maxn];
bool seen[maxn];
void dfs(int node) {
  seen[node] = 1;
  for (int son : G[node])
    if (!seen[son]) dfs(son);
  topo[code[node]].insert({out[node], node});
}
int process() {
  int answer = 0;
  while (!topo[1].empty()) {
    answer++;
    for (int i = 0; i < 2; i++) {
      while (!topo[i].empty() && topo[i].begin()->first == 0) {
        auto aux = *topo[i].begin();
        topo[i].erase(topo[i].begin());
        for (int son : R[aux.second]) {
          topo[code[son]].erase({out[son], son});
          out[son]--;
          topo[code[son]].insert({out[son], son});
        }
      }
    }
  }
  return answer;
}
int main() {
  ios::sync_with_stdio(false);
  int x, y;
  cin >> n >> m;
  for (int i = 1; i <= n; i++) cin >> code[i];
  for (int i = 1; i <= m; i++) {
    cin >> x >> y;
    x++;
    y++;
    G[x].push_back(y);
    R[y].push_back(x);
    in[y]++;
    out[x]++;
  }
  for (int i = 1; i <= n; i++)
    if (in[i] == 0) dfs(i);
  cout << process();
  return 0;
}
