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#include <bits/stdc++.h>

using namespace std;

map<long long, int> mapka;
long long bk = 0;
int bv = 0;

void put(long long k, int v) {
    auto it = mapka.lower_bound(k-bk);
    if (it != mapka.end()) {
        if (it->second < v-bv) {
            return;
        }
    }
    vector<long long> torem;
    while (it != mapka.begin()) {
        it--;
        if (it->second > v-bv) {
            torem.push_back(it->first);
        }
    }
    for (long long r : torem) {
        mapka.erase(r);
    }
    mapka[k-bk] = v-bv;
}

int get(long long k) {
    return mapka[k-bk] + bv;
}

void inckey(long long diff) {
    bk += diff;
}

void incval(int diff) {
    bv += diff;
}

pair<bool, pair<long long, int>> lower(long long k) {
    auto it = mapka.lower_bound(k-bk);
    if (it == mapka.end()) {
        return {false, {-1,-1}};
    }
    return {true, {it->first+bk, it->second+bv}};
}

void solve() {
    int n;
    cin >> n;
    vector<int> v(n);
    for (int i = 0; i < n; i++) {
        cin >> v[i];
    }
    put(0, 0);
    long long offset = 0;
    for (int i = 0; i < n; i++) {
        inckey(v[i]);
        incval(1);
        auto lp = lower(0);
        if (lp.first) {
            auto p = lp.second;
            put(0, p.second-1);
        }
    }

    auto lp = lower(0);
    if (lp.first) {
        auto p = lp.second;
        cout << p.second << "\n";
    } else {
        cout << "-1\n";
    }
}

int main() {
    ios_base::sync_with_stdio(false);
    cout.precision(15);
    cout << fixed;
    cin.tie(0);

    int T = 1;
    //cin >> T;
    for (int tst = 0; tst < T; tst++) {
        solve();
    }

    return 0;
}