#include <bits/stdc++.h>
using namespace std;
const int inf = 1 << 30;
const long long INF = 1ll << 60;
const double pi = acos(-1);
const double eps = 1e-9;
const long long mod = 1e9 + 7;
const int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
int n;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  int t = -1;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    if (t + 1 == x) t = x;
    if (t < x) {
      cout << i + 1 << endl;
      return 0;
    }
  }
  cout << -1 << endl;
}
