9-И-2 ex.1

# -*- coding: utf-8 -*-
"""
Created on Thu Nov 19 20:09:32 2020

@author: Sergey
"""

import math

def check(a,b,c):
    if (a + b >= c) and (a+c>=b) and (b+c>=a):
        return True
    else:
        return False

def sq(a,b,c):
    p=(a+b+c)/2
    S = math.sqrt(p*(p-a)*(p-b)*(p-c))
    return S

a,b,c,d=float(input()),float(input()),float(input()),float(input())
data=[a,b,c,d]
ans=-1
for i in range(4):
    for j in range(i+1,4):
        for k in range(j+1,4):
            if check(data[i],data[j],data[k]):
                ans = max(ans,sq(data[i],data[j],data[k]))
if ans == -1:
    print('Таких треугольников нет')
else:
    print(ans)