public static BigInteger sumSuffix ( long idx , int k ) {
  BigInteger t2 = new BigInteger ( 2 + "" ) ;
  BigInteger tIDX = new BigInteger ( idx + "" ) ;
  BigInteger k1 = new BigInteger ( k + "" ) ;
  BigInteger k2 = new BigInteger ( ( k + 1 ) + "" ) ;
  BigInteger n1 = ( k1 . multiply ( k2 ) ) . divide ( t2 ) ;
  BigInteger idx1 = new BigInteger ( idx + "" ) ;
  BigInteger idx2 = new BigInteger ( ( idx + 1 ) + "" ) ;
  BigInteger n2 = ( idx1 . multiply ( idx2 ) ) . divide ( t2 ) ;
  return n1 . subtract ( n2 ) . add ( tIDX ) ;
}

public static BigInteger sumSuffix ( long idx , int k ) { BigInteger t2 = new BigInteger ( 2 + " " ) ; BigInteger tIDX = new BigInteger ( idx + " " ) ; BigInteger k1 = new BigInteger ( k + " " ) ; BigInteger k2 = new BigInteger ( ( k + 1 ) + " " ) ; BigInteger n1 = ( k1 . multiply ( k2 ) ) . divide ( t2 ) ; BigInteger idx1 = new BigInteger ( idx + " " ) ; BigInteger idx2 = new BigInteger ( ( idx + 1 ) + " " ) ; BigInteger n2 = ( idx1 . multiply ( idx2 ) ) . divide ( t2 ) ; return n1 . subtract ( n2 ) . add ( tIDX ) ; }
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