public static void main ( String args [ ] ) {
  FastReader input = new FastReader ( ) ;
  PrintWriter out = new PrintWriter ( System . out ) ;
  int T = input . nextInt ( ) ;
  while ( T -- > 0 ) {
    int n = input . nextInt ( ) ;
    int k = input . nextInt ( ) ;
    int a [ ] = new int [ n ] ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      a [ i ] = input . nextInt ( ) ;
    }
    Arrays . sort ( a ) ;
    int v = a [ 0 ] + k ;
    int flag = 0 ;
    for ( int i = 1 ;
    i < n ;
    i ++ ) {
      int d = Math . abs ( v - a [ i ] ) ;
      if ( d > k ) {
        flag = 1 ;
        break ;
      }
    }
    if ( flag == 1 ) {
      out . println ( - 1 ) ;
    }
    else {
      out . println ( v ) ;
    }
  }
  out . close ( ) ;
}

public static void main ( String args [ ] ) { FastReader input = new FastReader ( ) ; PrintWriter out = new PrintWriter ( System . out ) ; int T = input . nextInt ( ) ; while ( T -- > 0 ) { int n = input . nextInt ( ) ; int k = input . nextInt ( ) ; int a [ ] = new int [ n ] ; for ( int i = 0 ; i < n ; i ++ ) { a [ i ] = input . nextInt ( ) ; } Arrays . sort ( a ) ; int v = a [ 0 ] + k ; int flag = 0 ; for ( int i = 1 ; i < n ; i ++ ) { int d = Math . abs ( v - a [ i ] ) ; if ( d > k ) { flag = 1 ; break ; } } if ( flag == 1 ) { out . println ( - 1 ) ; } else { out . println ( v ) ; } } out . close ( ) ; }
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