#include <bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using ll = long long int;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vd = vector<double>;
using vvd = vector<vd>;
using vvvd = vector<vvd>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using cdouble = complex<double>;
const double eps = 1e-7;
bool feq(double x, double y) { return abs(x - y) <= eps; }
bool inrange(ll x, ll t) { return x >= 0 && x < t; }
bool inrange(vll xs, ll t) {
  for (const auto &x : xs)
    if (!(x >= 0 && x < t)) return false;
  return true;
}
int ceillog2(ll x) { return int(ceil(log2(x))); }
int floorlog2(ll x) { return int(floor(log2(x))); }
template <class T>
T reversed(T container) {
  reverse(container.begin(), container.end());
  return container;
}
ll rndf(double x) { return (ll)(x + (x >= 0 ? 0.5 : -0.5)); }
ll floorsqrt(ll x) {
  ll m = (ll)sqrt((double)x);
  return m + (m * m <= x ? 0 : -1);
}
ll ceilsqrt(ll x) {
  ll m = (ll)sqrt((double)x);
  return m + (x <= m * m ? 0 : 1);
}
ll rnddiv(ll a, ll b) { return (a / b + (a % b * 2 >= b ? 1 : 0)); }
ll ceildiv(ll a, ll b) { return (a / b + (a % b == 0 ? 0 : 1)); }
ll gcd(ll m, ll n) {
  if (n == 0)
    return m;
  else
    return gcd(n, m % n);
}
ll lcm(ll m, ll n) { return m * n / gcd(m, n); }
void failed() {
  cout << -1 << endl;
  exit(0);
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ;
  int n, m;
  cin >> n >> m;
  vll v(n);
  for (int i = 0; i < int(n); i++) cin >> v[i];
  vvi lst(n);
  vvi eid(n);
  for (int i = 0; i < int(m); i++) {
    int s, t;
    cin >> s >> t;
    s--;
    t--;
    lst[s].push_back(t);
    lst[t].push_back(s);
    eid[s].push_back(i);
    eid[t].push_back(i);
  }
  failed();
}
