#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N = 1e6 + 5;
const int mod = 998244353;

int n;
//int a[N];
void solve() {
    cin >> n;
    int ans = 1500;
    for (int i = 1; i <= n; i++) {
        int x;
        cin >> x;
        ans += x;
        if (x >= 4000) {
            cout << i << '\n';
            return;
        }
    }
    cout << -1 << '\n';
}
signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    int T = 1;
    //cin >> T;
    while (T--)
        solve();
}