import math

a, b, c = map(int, input().split())

# Calculate the semi-perimeter
s = (a + b + c) / 2.0

# Calculate the area using Heron's formula
area = math.sqrt(s * (s - a) * (s - b) * (s - c))

# The expected area is 1/4 of the original area
expected = area / 4.0

# Print with sufficient precision
print("{0:.10f}".format(expected))