import math

while True:
    try:
        a,l,x=map(int, input().split())
        temp=(l+x)/2
    except EOFError:
        break

    def heron(i,j,k):
        d = (i+j+k)/2
        return math.sqrt(d*(d-i)*(d-j)*(d-k))

    print((str(heron(a,l,l)+heron(l,temp,temp)*2)))
