from decimal import Decimal
def make_divisors(n):
    lower_divisors , upper_divisors = [], []
    i = 1
    while i*i <= n:
        if n % i == 0:
            lower_divisors.append(i)
            if i != n // i:
                upper_divisors.append(n//i)
        i += 1
    return lower_divisors + upper_divisors[::-1]
x, y, z, w = map(int, input().split())
d = make_divisors(x)
for i in d:
    oth = x // i
    if i < z or oth < y:
        continue
    try:
        if Decimal(str(y ** 2 + i ** 2)) ** Decimal("0.5") - Decimal(str(z ** 2 + oth ** 2)) ** Decimal("0.5") == Decimal(str(w)):
            print(x - y * i / 2 - z * oth / 2 - (i - z) * (oth - y) / 2)
            break
    except:
        continue