/* Check cf5-opt.vim defs.
VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
VIM-: let g:cppflags=g:Iboost.g:Itbb
VIM-: let g:ldflags=g:Lboost.g:Ltbb.g:tbbmalloc.g:tbbmproxy
VIM-: let g:ldlibpath=g:Bboost.g:Btbb
VIM: let g:argv=""
*/
#include <iostream>
#include <vector>
#include <stdexcept>

int main ( void )
{try{

	//
	//	Read length.
	//
	long long n;
	std::cin >> n;
	if ( n < 4 )
		throw std::runtime_error( "Input length is not enough." );
	//
	//	Read seq.
	//
	typedef std::vector<int> arr_type;
	arr_type arr(n);
	for ( int& i: arr )
		std::cin >> i;
	//
	//	Solve it.
	//
	long long s = 0;
	for ( long long i: arr )
		s += i;
	long long d = arr[1]-arr[0];
	if ( d != arr[2]-arr[1] && d != arr[3]-arr[2])
		d = arr[2]-arr[1];
	long long idx = (n+1)*n/2-(s-n*arr[0])/d;
	//
	//
	//
	long long missing = arr[0]+idx*d;
	std::cout << missing;

	return 0;
}
catch ( const std::exception& e )
{
	std::cerr << std::endl
			<< "std::exception(\"" << e.what() << "\")." << std::endl;
	return 2;
}
catch ( ... )
{
	std::cerr  << std::endl
			<< "unknown exception." << std::endl;
	return 1;
}}

