def find_least_common_number(a, b, c):
    """Find least common number in the three given sorted array.
    Return -1 if there is no common number.

    Time:  O(n)
    Space: O(1)
    """
    i = j = k = 0
    while i < len(a) and j < len(b) and k < len(c):
        x, y, z = a[i], b[j], c[k]
        if x == y == z:
            return x
        m = max(x, y, z)
        if x < m:
            i += 1
        if y < m:
            j += 1
        if z < m:
            k += 1
    return -1