#include<bits/stdc++.h>
using std::cin;
using std::cout;
using std::endl;
int main(){
    long long a,b,c; cin>>a>>b>>c;
    //a^2+b^2-2abcosx=c^2
    double cosin=(a*a+b*b-c*c)/2.0/a/b;
    double sin=sqrt(1-cosin*cosin);
    double area=a*b*sin/2.0;
    double ans=area/4.0;
    printf("%.10lf\n",ans);
}