def rsearch(li, n, start, end):
    index = (start + end) // 2
    if li[index] == n:
        return index
    elif li[index] > n:
        return rsearch(li, n, start, index)
    elif li[index] < n:
        return rsearch(li, n, index, end)

li = [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
print(rsearch(li, 13, 0, len(li)))