#include <iostream>
#include <algorithm>
#include <cstdio>
using namespace std;

int main(){
	double a,b,c,p,ans;
	cin>>a>>b>>c;
	p = (a + b + c) / 2.0;
	ans = sqrt(p * (p - a) * (p - b) * (p - c) * 1.0);
	printf("%.1f\n",ans);
	return 0;
}