E10

#include<bits/stdc++.h>
using namespace std;

int arr[3000000];
int pos[3000001];
vector<int> ans;
queue<int> free_pos;
bitset<32000000> used;
char res[32000000];
// 32000000
void Pop(int i) {
    ans.push_back(i + 1);
}

void Push() {
    ans.push_back(0);
}

int main() {
    ios_base::sync_with_stdio(false);
    ofstream cout("output.bin", std::ios::binary | std::ios::out);
    ifstream cin("input.bin", std::ios::binary | std::ios::in);
    cout.tie(nullptr);
    cin.tie(nullptr);
    int n = 0;
    int maxx = 0;
    cin.read((char*)&n, 4);
    for (int i = 0; i < n; ++i) {
        cin.read((char*)(arr + i), 4);
        maxx = max(arr[i], maxx);
    }
    memset(pos, -1, sizeof(pos));
    for (int i = 0; i < n; ++i) {
        free_pos.push(i);
        if (arr[i] == 0) {
            continue;
        }
        if (arr[i] < i + 1) {
            int minus1 = -1;
            cout.write((char*)&minus1, 4);
            cin.close();
            cout.close();
            return 0;
        }
        pos[arr[i]] = i;
    }
    ans.reserve(3 * n);
    int last = 0;
    for (int x = 1; x <= maxx; ++x) {
        if (pos[x] == -1) {
            if (last == n) {
                while (used[free_pos.front()]) free_pos.pop();
                Pop(free_pos.front());
                --last;
            }
            Push();
        } else {
            if (pos[x] > last) {
                int minus1 = -1;
                cout.write((char*)&minus1, 4);
                cin.close();
                cout.close();
                return 0;
            }
            if (pos[x] < last) {
                Pop(pos[x]);
                --last;
            }
            Push();
            used[pos[x]] = true;
        }
        ++last;
    }
    for (int i = 0; i < last; ++i) {
        if (arr[i] == 0) {
            Pop(i);
        }
    }
    int sz = ans.size();
    char* ptr = (char*)&sz;
    for (int i = 0; i < 4; ++i) {
        res[i] = ptr[i];
    }
    int cur = 4;
    for (int x : ans) {
        ptr = (char*)&x;
        for (int i = 0; i < 4; ++i) {
            res[cur++] = ptr[i];
        }
    }
    cout.write(res, cur);
    cin.close();
    cout.close();
    return 0;
}