#include <bits/stdc++.h>
using namespace std;
int main(){
	int a,b,c;
	cin>>a>>b>>c;
	if(a*a+b*b==c*c)printf("%.1f",a*b/2.0);
	if(b*b+c*c==a*a)printf("%.1f",b*c/2.0);
	if(a*a+c*c==b*b)printf("%.1f",a*b/2.0);
	return 0;
}
/**************************************************************
	Problem: 1047
	User: panyuchen
	Language: C++
	Result: Wrong Answer
****************************************************************/