public static int pell ( int n ) {
  if ( n <= 2 ) return n ;
  int a = 1 ;
  int b = 2 ;
  int c ;
  for ( int i = 3 ;
  i <= n ;
  i ++ ) {
    c = 2 * b + a ;
    a = b ;
    b = c ;
  }
  return b ;
}

public static int pell ( int n ) { if ( n <= 2 ) return n ; int a = 1 ; int b = 2 ; int c ; for ( int i = 3 ; i <= n ; i ++ ) { c = 2 * b + a ; a = b ; b = c ; } return b ; }
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