#include <bits/stdc++.h>
#include <math.h>
using namespace std;
int main()
{
double a,b,c,ans,s;
while(scanf("%lf %lf %lf",&a,&b,&c) == 3 )
{
s=(a+b+c)/2;
ans=sqrt(s*(s-a)*(s-b)*(s-c));
if(ans>0)
printf("%.3lf\n",ans*(4.0/3.0));
else
{
ans=-1;
printf("%.3lf\n",ans);
}
}
return 0;
}
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