def find_missing(elements):
    if len(elements) == 1:
        return elements[0]
    a1 = elements[0]
    d = elements[1] - elements[0]
    if d != elements[2] - elements[1]:
        d = elements[2] - elements[1]
    for i in range(1, len(elements)):
        a = a1 + (d * (i - 1))
        if a not in elements:
            return a