Solution

#include <iostream>
#include <vector>
#include <cstdio>
#include <algorithm>

using namespace std;

int main() {
    int N; cin >> N;

    vector<bool> used(2 * N);
    vector<int> A(N);
    for (int i = 0; i < N; i++) {
        cin >> A[i];
        A[i]--;
        used[A[i]] = true;
    }
    sort(A.begin(), A.begin() + N / 2);
    sort(A.begin() + N / 2, A.end());
    rotate(A.begin(), A.begin() + N / 2, A.end());

    vector<int> B;
    for (int i = 0; i < 2 * N; i++) {
        if (!used[i]) {
            B.push_back(i);
        }
    }

    int res = 0;
    for (int i = N / 2, j = N / 2; i < N; i++, j++, res++) {
        while (j < N && B[j] < A[i]) {
            j++;
        }
        if (j == N) {
            break;
        }
    }
    for (int i = N / 2 - 1, j = N / 2 - 1; i >= 0; i--, j--, res++) {
        while (j >= 0 && B[j] > A[i]) {
            j--;
        }
        if (j == -1) {
            break;
        }
    }
    cout << res << endl;

    return 0;
}