iOverlapLen (a1, a2, b1, b2)

Write the function iOverlapLen that computes the length of overlap of two closed intervals. It takes 4 numbers (ints or floats) a1, a2, b1, b2 that describe the two closed intervals [a1,a2] and [b1,b2] of the real number line, and returns the length of the intersection of these two intervals, always as a float. If a1>a2 , then the interval [a1,a2] is empty. If b1>b2 , then the interval [b1,b2] is empty.