#include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;
double area(double a,double b,double c){
double p=(a+b+c)/2.0;
return sqrt(p*(p-a)(p-b)(p-c));
}
int main() {
double a,b,c;
cin>>a>>b>>c;
printf("%.2f",area(a,b,c));
return 0;
}
