shashika - given the error message, it sounds like you are trying to use S (which is negative) as an index into array. Perhaps something like
someArray(S)
As the error message states, the index must be a positive integer or logical. Your S is negative so using this value to access an element in an array does not make sense. You need either to guard against using a negative index or ensure that the code which initializes S only does so with valid positive integers.
