#include<iostream>
#include<iomanip>
#include<cmath>
#include<cstdio>
using namespace std;
int main()
{
    unsigned long long a,b,c;
    long double p,P;
    scanf("%lld%lld%lld",&a,&b,&c);
    p=(a+b+c)/2;
    P=sqrt((p-a)*(p-b)*(p-c)*p);
    printf("%.1Lf",P);
    return 0;
}