from sys import stdin
import math
import re
import queue

input = stdin.readline

MOD = 1000000007
INF = 122337203685477580

 
def heron(a, b, c):
	d = (a + b + c) / 2
	return math.sqrt(d * (d - a) * (d - b) * (d - c))
def solve():
    a = list(map(float,input().split()))
       
    res = heron(a[0],a[1],a[2])/4
    print(res)
   


if __name__ == '__main__':
    solve()