def fac(n):
    if n == 0:
        return 1
    else:
        return n* fac(n - 1)
def C(n, m):
    return fac(n)// (fac(m)* fac(n - m))
i, j, n, m = map(int, input().split())
print(C(i, j) + C(n, m))