import math

def compute_volume(R, H, D):
    a = D / (2 * R)
    if a >= 1.0:
        return 0.0  # No intersection volume
    
    # Pre-calculate frequently used constants
    two_R_squared = 2 * R * R
    four_R_squared = 4 * R * R
    D_squared = D * D
    D_div_2 = D / 2

    def integrand(t):
        arg = D / (2 * R * t)
        arg_clipped = max(min(arg, 1.0), -1.0)
        acos_val = math.acos(arg_clipped)
        term1 = two_R_squared * (t ** 2) * acos_val
        sqrt_val = math.sqrt(four_R_squared * t * t - D_squared)
        term2 = D_div_2 * sqrt_val
        return term1 - term2

    # Use Simpson's rule with a large number of intervals
    N = 1000000  # Must be even
    a_t = a
    b_t = 1.0
    h = (b_t - a_t) / N

    integral = 0.0
    for i in range(N + 1):
        t = a_t + i * h
        if i == 0 or i == N:
            weight = 1
        elif i % 2 == 1:
            weight = 4
        else:
            weight = 2
        integral += integrand(t) * weight

    integral *= h / 3
    volume = H * integral
    return volume

# Read input
R, H, D = map(float, input().split())

# Compute the volume
volume = compute_volume(R, H, D)

# Output with sufficient precision
print("{0:.9f}".format(volume))