#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

int main()
{
    ifstream in;
    ofstream out;
    in.open( "cardgame.in" );
    out.open( "cardgame.out" );

    int n;
    in >> n;

    vector<int> elsie(n);
    vector<int> bessie(2*n, 0); // -1 = doesn't have, 0 = unused, 1 = lost, 2 = won
    for( int i = 0; i < n; i++ )
    {
        in >> elsie[i];
        bessie[--elsie[i]] = -1;
    }

    vector<int> lower;
    vector<int> higher;

    for( int i = 0, k = 0; i < 2*n; i++ )
    {
        if( bessie[i] == 0 )
        {
            if( k < n/2 )
            {
                lower.push_back( i );
                k++;
            } else
                higher.push_back( i );
        }
    }

    for( int i = 0; i < n/2; i++ )
    {
        if( elsie[i] > higher.back() )
        {
            bessie[higher.front()] = 1;
            higher.erase( higher.begin() );
        } else
        {
            int j = 0;
            for( j = 0; j < higher.size() && higher.at( j ) < elsie[i]; j++ );
            bessie[higher.at( j )] = 2;
            higher.erase( higher.begin()+j );
        }
    }

    for( int i = n/2; i < n; i++ )
    {
        if( elsie[i] < lower.front() )
        {
            bessie[lower.back()] = 1;
            lower.erase( lower.end()-1 );
        } else
        {
            int j = 0;
            for( j = lower.size()-1; j >= 0 && lower.at( j ) > elsie[i]; j-- );
            bessie[lower.at( j )] = 2;
            lower.erase( lower.begin()+j );
        }
    }

    int ans = 0;
    for( int i = 0; i < 2*n; i++ )
        if( bessie[i] == 2 )
            ans++;

    out << ans << endl;

    in.close();
    out.close();
    return 0;
}