mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

    a, b, c = map(int, input().split())
    s = (a+b+c) / 2
    print(((s * (s-a) * (s-b) * (s-c)) ** 0.5) / 4)


if __name__ == '__main__':
    main()