def bin_search(a, x):
    n = len(a)

    def get(i):
        if i == 0:
            return float('-inf')
        elif i == n + 1:
            return float('inf')
        else:
            return a[i - 1]

    l, r = 0, len(a) + 1
    while l < r - 1:
        m = l + (r - l) / 2
        if get(m) <= x:
            l = m
        else:
            r = m
    if get(l) == x:
        return l - 1
    else:
        return -1


assert bin_search([1, 2, 3], 3) == 2
assert bin_search([], 3) == -1
assert bin_search([1, 3], 2) == -1