#include <bits/stdc++.h>
using namespace std;
using li = long long;
using ld = long double;
void solve(bool);
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << fixed << setprecision(20);
  solve(true);
  return 0;
}
const int maxn = (int)1e5 + 5;
int n, a[maxn];
map<int, int> m;
void solve(bool __attribute__((unused)) read) {
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
    if (a[i] > 0 && !m.count(a[i] - 1)) {
      cout << i + 1 << "\n";
      return;
    }
    m[a[i]] = 1;
  }
  cout << -1 << "\n";
  return;
}
