from sys import stdin
from math import sqrt

def area(a, b, c):
        d = (a + b + c) / 2.0
        return sqrt(d * (d - a) * (d - b) * (d - c))

for line in stdin:
        (a, l, x) = map(int, line.split())
        print "%.6f" % (area(a, l, l) + 2 * area(l, (l + x) / 2.0, (l + x) / 2.0))