(Reuters) -Merck & Co posted better-than-expected second-quarter sales on Tuesday on strong demand for its two top-selling products, cancer immunotherapy Keytruda and human papillomavirus (HPV) vaccine Gardasil. Shares of Merck rose about 2% at $108.45 in premarket trading after the company also raised its full-year revenue forecast. Merck said Keytruda sales for the quarter jumped 19% to $6.3 billion, surpassing analysts' average estimate of $5.9 billion.