Randomly select a number from the following five numbers -2, -1, -1/2, 1,2, and denote it as a. Suppose the system of inequalities with respect to X: {2x+7 geq 9 and X-a<0} has no solution, and the solution of a/(2x-3) +(a-2)/(2x-3) =-1 is a positive non integer fraction. Find the sum of all valid values of a
