#include<bits/stdc++.h>
using namespace std;
float mianji(int a,int b,int c){
	int p=(a+b+c)/2;
	float s=sqrt(p*(p-a)*(p-b)*(p-c));
	return s;
}
int main(){
	float a,b,c;
	cin>>a>>b>>c;
	printf("%.1f",mianji(a,b,c));
	cout<<endl;
	return 0;
}