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

using ll = long long;

#define TYPE_OF( VAR ) remove_const<remove_reference<decltype( VAR )>::type >::type
#define UNTIE ios_base::sync_with_stdio( false ); cin.tie( nullptr ) 
#define CIN( LL , A ) LL A; cin >> A 
#define ASSERT( A , MIN , MAX ) assert( MIN <= A && A <= MAX ) 
#define CIN_ASSERT( A , MIN , MAX ) CIN( TYPE_OF( MAX ) , A ); ASSERT( A , MIN , MAX ) 
#define FOREQ( VAR , INITIAL , FINAL ) for( TYPE_OF( FINAL ) VAR = INITIAL ; VAR <= FINAL ; VAR ++ )  
#define QUIT return 0 
#define RETURN( ANSWER ) cout << ( ANSWER ) << "\n"; QUIT 

#define CHECK_REDUNDANT_INPUT string VARIABLE_FOR_CHECK_REDUNDANT_INPUT = ""; cin >> VARIABLE_FOR_CHECK_REDUNDANT_INPUT; assert( VARIABLE_FOR_CHECK_REDUNDANT_INPUT == "" ); assert( ! cin );
// #define CHECK_REDUNDANT_INPUT string VARIABLE_FOR_CHECK_REDUNDANT_INPUT = ""; getline( cin , VARIABLE_FOR_CHECK_REDUNDANT_INPUT ); assert( VARIABLE_FOR_CHECK_REDUNDANT_INPUT == "" ); assert( ! cin );

int main()
{
  constexpr const int bound = 100000;
  CIN_ASSERT( N , 1 , bound );
  int count[bound + 1] = {};
  int Ai;
  FOREQ( i , 1 , N ){
    cin >> Ai;
    ASSERT( Ai , 0 , bound );
    count[Ai]++;
  }
  CHECK_REDUNDANT_INPUT;
  int count_a;
  FOREQ( a , 0 , bound ){
    count_a = count[a];
    if( count_a == 0 ){
      RETURN( a );
    } else if( count_a == 1 ){
      if( a * 2 + 1 == N ){
	RETURN( a );
      }
      FOREQ( b , a + 1 , bound ){
	count_a = count[b];
	if( count_a == 0 ){
	  if( b * 2 - 1 == N ){
	    RETURN( b - 1 );
	  } else {
	    RETURN( b );
	  }
	} else if( count_a == 1 ){
	  RETURN( b );
	}
      }
    }
  }
  QUIT;
}