rule prohibits the admission of evidence to contradict or add to the terms of a clear and unambiguous contract."' Thompson, supra, at 709, quoting Hansen v. DHL Laboratories (1994), 316 S.C. 505, 508, 450 S.E.2d 624, affirmed (1995), 319 S.C. 79, 459 S.E.2d 850. Accord Citicasters Co. v. Bricker & Eckler, L.L.P., 149 Oh