A JUDGE threatened to increase a thief's sentence when he tried to convince the court that his knife was actually an umbrella. 'If you waste my time I'll increase your sentence,' Mr Justice Duffy warned Yeung Chi-chiu, 28, yesterday. On April 23, Yeung pleaded guilty to theft and possession of an offensive weapon and was sentenced to 15 months in prison. He appeared in the High Court yesterday to appeal against both the conviction and the sentence. Mr Justice Duffy was unsympathetic and said if he had been sentencing Yeung, who is an illegal immigrant, he would have given him 21/2 years in prison. 'I will not have people coming down from China and committing crimes of this sort,' the judge told him. When the judge asked Yeung if he still wanted to continue with his appeal, the thief replied, 'All right, forget it', and left the dock. Yeung entered Hong Kong on the morning of the crime and was arrested in Tai Po on February 16 carrying a 27-centimetre knife. The construction worker confessed he had spent his first morning in Hong Kong stealing clothing from various flats. He claimed to have come to the territory to earn money for his father's medical expenses. Yesterday, Yeung tried to convince the judge that police had mistaken his umbrella for a knife. Mr Justice Duffy rejected the claim. At his April hearing Yeung confessed he was carrying the blade.