import math
while True :
    try :
        a, l, x = map(int, input().split())
    except EOFError :
        break

    HC = math.sqrt(l**2 - a**2 / 4)
    I = math.sqrt(((l+x)/2)**2 - l**2 / 4)
    S1 = a * HC / 2
    S2 = l * I / 2
    print('{:.8f}'.format(S1 + S2*2))
