#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;

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