#include <bits/stdc++.h>
using namespace std;
long long power(long long x, long long y);
long long mpower(long long x, long long y, long long p);
long long modInv(long long a, long long m);
long long gcdExtended(long long a, long long b, long long *x, long long *y);
bool isPrime(long long n);
vector<long long> A[300001];
long long vis[300001], lvl[300001];
void BFS(long long x) {
  long long i;
  vis[x] = 1;
  lvl[x] = 0;
  queue<long long> Q;
  Q.push(x);
  while (Q.size()) {
    x = Q.front();
    Q.pop();
    for (i = 0; i < A[x].size(); i++) {
      if (vis[A[x][i]] == 0) {
        Q.push(A[x][i]);
        vis[A[x][i]] = 1;
        lvl[A[x][i]] = lvl[x] + 1;
      }
    }
  }
}
long long BFS2(long long x) {
  long long i, c = 1;
  vis[x] = 1;
  queue<long long> Q;
  Q.push(x);
  while (Q.size()) {
    x = Q.front();
    Q.pop();
    for (i = 0; i < A[x].size(); i++) {
      if (vis[A[x][i]] == 0 && lvl[A[x][i]] == 1) {
        Q.push(A[x][i]);
        vis[A[x][i]] = 1;
        lvl[A[x][i]] = lvl[x] + 1;
        c++;
      }
    }
  }
  return c;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long t, i, j, k, l, h, p, q, r, x, y, z, a, b, c, d, f, n, m,
      M = 1000000007;
  char ch, ch2, ch3;
  string s, s1, s2, s3;
  vector<long long> v, w;
  vector<long long>::iterator itrv;
  unordered_map<long long, long long> N;
  set<long long> S;
  set<long long>::iterator itr;
  cin >> n >> m;
  long long X[n + 1];
  for (i = 1; i <= n; i++) {
    cin >> X[i];
  }
  for (i = 0; i < m; i++) {
    cin >> x >> y;
    A[y].push_back(x);
  }
  BFS(X[n]);
  x = 0;
  for (i = 1; i <= n; i++) {
    if (lvl[i] == 1) {
      v.push_back(i);
    }
  }
  if (v.empty()) {
    cout << 0 << endl;
    return 0;
  }
  for (i = 0; i <= n; i++) {
    vis[i] = 0;
  }
  for (i = 0; i < v.size(); i++) {
    if (vis[v[i]] == 0) {
      x = max(x, BFS2(v[i]));
    }
  }
  cout << x << endl;
  return 0;
}
long long modInv(long long a, long long m) {
  long long x, y;
  long long g = gcdExtended(a, m, &x, &y);
  long long res = (x % m + m) % m;
  return res;
}
long long gcdExtended(long long a, long long b, long long *x, long long *y) {
  if (a == 0) {
    *x = 0, *y = 1;
    return b;
  }
  long long x1, y1;
  long long gcd = gcdExtended(b % a, a, &x1, &y1);
  *x = y1 - (b / a) * x1;
  *y = x1;
  return gcd;
}
long long mpower(long long x, long long y, long long p) {
  long long res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long power(long long x, long long y) {
  long long res = 1;
  while (y > 0) {
    if (y & 1) res = res * x;
    y = y >> 1;
    x = x * x;
  }
  return res;
}
bool isPrime(long long n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  long long p = sqrt(n);
  for (int i = 5; i <= p; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0) return false;
  return true;
}
