#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
using mint = modint998244353;

int N, M, H[101010];
vector<int> G[101010];

void bfs(int start, int goal, int s) {
  ll ans1 = 0;
  queue<tuple<int, ll, bool>> que1;
  que1.push({start, 0, false});
  map<pair<int, int>, bool> vis1;
  bool flag1 = false;
  while (!que1.empty()) {
    auto &[u, c, f] = que1.front(); que1.pop();
    if (u == goal) {
      flag1 = true;
      ans1 = max(ans1, c);
      continue;
    }
    for (auto v: G[u]) {
      if (s == 0 && u > v) continue;
      if (s == 1 && u < v) continue;
      if (f && H[u] < H[v]) continue;
      ll c1 = c;
      bool f1 = f;
      if(vis1[{min(u, v), max(u, v)}]) continue;
      vis1[{min(u, v), max(u, v)}] = true;
      if (H[u] < H[v]) {
        c1 += abs(H[v] - H[u]);
        f1 = true;
      } else {
        f1 = false;
      }
      que1.push({v, c1, f1});
    }
  }
  cout << (flag1 ? ans1: -1) << endl;
}

int main() {
  cin >> N >> M;
  for (int i = 1; i <= N; i++) cin >> H[i];
  for (int i = 1; i <= M; i++) {
    int x, y; cin >> x >> y;
    G[x].push_back(y);
    G[y].push_back(x);
  }
  bfs(1, N, 0);
  bfs(N, 1, 1);
  return 0;
}