#include<iostream>
#include<cmath>
using namespace std;
#define p(a,b,c) (a+b+c)/2
#define S(p,a,b,c) sqrt(p*(p-a)*(p-b)*(p-c))
int main()
{
double a, b, c;
cin >> a >> b >> c;
double p, S;
p = p(a, b, c);
S = S(p, a, b, c);
printf("%.3lf", S);
}
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