public static void main ( String [ ] args ) {
  Scanner cin = new Scanner ( System . in ) ;
  int n = cin . nextInt ( ) ;
  int k = cin . nextInt ( ) ;
  ArrayList < Integer > [ ] a = new ArrayList [ 100001 ] ;
  for ( int i = 0 ;
  i < 100001 ;
  i ++ ) {
    a [ i ] = new ArrayList < Integer > ( ) ;
  }
  for ( int i = 1 ;
  i <= k ;
  i ++ ) {
    int x = cin . nextInt ( ) ;
    a [ x ] . add ( i ) ;
  }
  int ans = 0 ;
  for ( int i = 1 ;
  i <= n ;
  i ++ ) {
    for ( int j = i - 1 ;
    j <= i + 1 ;
    j ++ ) {
      if ( j > 0 && j < n + 1 ) {
        if ( i == j && a [ i ] . isEmpty ( ) ) {
          ans ++ ;
        }
        if ( i != j && ! a [ i ] . isEmpty ( ) && ! a [ j ] . isEmpty ( ) && a [ i ] . get ( 0 ) > a [ j ] . get ( a [ j ] . size ( ) - 1 ) ) {
          ans ++ ;
        }
        if ( ( i != j ) && ( ( a [ i ] . isEmpty ( ) && ! a [ j ] . isEmpty ( ) ) || ( ! a [ i ] . isEmpty ( ) && a [ j ] . isEmpty ( ) ) || ( a [ i ] . isEmpty ( ) && a [ j ] . isEmpty ( ) ) ) ) {
          ans ++ ;
        }
      }
    }
  }
  System . out . println ( ans ) ;
}

public static void main ( String [ ] args ) { Scanner cin = new Scanner ( System . in ) ; int n = cin . nextInt ( ) ; int k = cin . nextInt ( ) ; ArrayList < Integer > [ ] a = new ArrayList [ 100001 ] ; for ( int i = 0 ; i < 100001 ; i ++ ) { a [ i ] = new ArrayList < Integer > ( ) ; } for ( int i = 1 ; i <= k ; i ++ ) { int x = cin . nextInt ( ) ; a [ x ] . add ( i ) ; } int ans = 0 ; for ( int i = 1 ; i <= n ; i ++ ) { for ( int j = i - 1 ; j <= i + 1 ; j ++ ) { if ( j > 0 && j < n + 1 ) { if ( i == j && a [ i ] . isEmpty ( ) ) { ans ++ ; } if ( i != j && ! a [ i ] . isEmpty ( ) && ! a [ j ] . isEmpty ( ) && a [ i ] . get ( 0 ) > a [ j ] . get ( a [ j ] . size ( ) - 1 ) ) { ans ++ ; } if ( ( i != j ) && ( ( a [ i ] . isEmpty ( ) && ! a [ j ] . isEmpty ( ) ) || ( ! a [ i ] . isEmpty ( ) && a [ j ] . isEmpty ( ) ) || ( a [ i ] . isEmpty ( ) && a [ j ] . isEmpty ( ) ) ) ) { ans ++ ; } } } } System . out . println ( ans ) ; }
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dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (type_identifier) dimensions: (dimensions_expr (decimal_integer_literal))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (decimal_integer_literal)) update: (update_expression (identifier)) body: (block (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (object_creation_expression type: (generic_type (type_identifier) (type_arguments (type_identifier))) arguments: (argument_list)))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (expression_statement (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list (identifier)))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (identifier) right: 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(binary_expression left: (identifier) right: (identifier)) right: (unary_expression operand: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list)))) right: (unary_expression operand: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list)))) right: (binary_expression left: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list (decimal_integer_literal))) right: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal))))))) consequence: (block (expression_statement (update_expression (identifier))))) (if_statement 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