#include <bits/stdc++.h>
template <typename T>
void MACRO_VAR_Scan(T& t) {
  std::cin >> t;
}
template <typename First, typename... Rest>
void MACRO_VAR_Scan(First& first, Rest&... rest) {
  std::cin >> first;
  MACRO_VAR_Scan(rest...);
}
template <typename T>
void MACRO_VEC_ROW_Init(int n, T& t) {
  t.resize(n);
}
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Init(int n, First& first, Rest&... rest) {
  first.resize(n);
  MACRO_VEC_ROW_Init(n, rest...);
}
template <typename T>
void MACRO_VEC_ROW_Scan(int p, T& t) {
  std::cin >> t[p];
}
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Scan(int p, First& first, Rest&... rest) {
  std::cin >> first[p];
  MACRO_VEC_ROW_Scan(p, rest...);
}
template <typename T>
inline T CHMAX(T& a, const T b) {
  return a = (a < b) ? b : a;
}
template <typename T>
inline T CHMIN(T& a, const T b) {
  return a = (a > b) ? b : a;
}
void CHECKTIME(std::function<void()> f) {
  auto start = std::chrono::system_clock::now();
  f();
  auto end = std::chrono::system_clock::now();
  auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start))
                 .count();
  std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n";
}
template <class T>
std::vector<std::vector<T>> VV(int n) {
  return std::vector<std::vector<T>>(n);
}
template <class T>
std::vector<std::vector<T>> VV(int n, int m, T init = T()) {
  return std::vector<std::vector<T>>(n, std::vector<T>(m, init));
}
template <typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<ll, ll>;
using PAIRLL = std::pair<ll, ll>;
constexpr ll INFINT = 1 << 30;
constexpr ll INFINT_LIM = (1LL << 31) - 1;
constexpr ll INFLL = 1LL << 60;
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);
constexpr double EPS = 1e-9;
constexpr ll MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
signed main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);
  ;
  ll n, m;
  MACRO_VAR_Scan(n, m);
  ;
  std::vector<ll> e(n);
  for (auto& i : e) std::cin >> i;
  ;
  std::vector<ll> a, b;
  MACRO_VEC_ROW_Init(m, a, b);
  for (ll i = 0; i < m; ++i) {
    MACRO_VEC_ROW_Scan(i, a, b);
  };
  struct Node {
    ll type;
    ll cnt[2] = {};
    std::vector<ll> to;
  };
  std::vector<Node> g(n);
  for (ll i = 0; i < ll(n); ++i) {
    g[i].type = e[i];
  }
  for (ll i = 0; i < ll(m); ++i) {
    g[a[i]].to.emplace_back(b[i]);
    ++g[b[i]].cnt[g[a[i]].type];
  }
  std::queue<ll> que[2];
  for (ll i = 0; i < ll(n); ++i) {
    if (g[i].cnt[0] == 0 && g[i].cnt[1] == 0) {
      que[g[i].type].push(i);
    }
  }
  ll nowq = 0, ans = 0;
  while (!que[0].empty() || !que[1].empty()) {
    if (que[nowq].empty()) {
      nowq = 1 - nowq;
      continue;
    }
    if (nowq == 1) ++ans;
    while (!que[nowq].empty()) {
      ll now = que[nowq].front();
      que[nowq].pop();
      for (auto& to : g[now].to) {
        --g[to].cnt[g[now].type];
        if (g[to].cnt[0] == 0 && g[to].cnt[1] == 0) {
          que[g[to].type].push(to);
        }
      }
    }
  }
  std::cout << ans;
  std::cout << "\n";
  ;
  return 0;
}
