'''
Дан список чисел, нужно вывести элемент у которого нет дупбиката

1) Решение через count
2) Решение через словарь
3) Решение через доп список

'''


def count_posibility(arr):
    for el in arr:

        if arr.count(el) == 1:
            return el

    return None


def dictionary_posibility(arr):
    dictionary = {}

    for element in arr:

        if element not in dictionary.keys():
            dictionary[element] = 0
        else:
            dictionary[element] += 1

    for element in dictionary.values():

        if element == 0:
            return element

    return None


def tuple_posibility(arr: list):
    stack = []
    arr.sort()
    for element in arr:

        if element not in stack:

            stack += [element]

        elif (element not in stack) and (type(tuple) in stack):

            for el in stack:

                if type(el) == list:

                    if el[0] == element:

                        index = stack.index(el)
                        i = 0
                        while i < len(stack):

                            if i == index:
                                stack.pop(index)
                                break

                            i += 1
        else:

            result = stack.pop()
            print(type(tuple[element]))
            stack.append([element])

    x_stack = []

    for element in stack:

        if type(element) == list:
            x_stack += [element[0]]

    y_stack = []

    for el in stack:

        if type(el) == list:

            y_stack += [el[0]]
        else:
            y_stack += [el]

    for element in y_stack:

        if element not in x_stack:

            return element
        else:

            continue


arr = list(map(int, input().split()))

print(tuple_posibility(arr))