def factorial(number):
    res = 1

    for i in range(2, number + 1):
        res *= i

    return res


def comb_num(number, n_fact, k):
    res = 0
    res = n_fact / (factorial(k) * factorial(number - k))

    return res * k

input_line = input()
a, b, k, c = input_line.split()
a = int(a)
b = int(b)
c = int(c)
k = int(k)
k_fact = factorial(k)
res = 0

if c == b or c == a:
    if a != b:
        for i in range(1, k+1):
            res += comb_num(k, k_fact, i)
    else:
        res = k

print(int(res))