#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);
	return 0;
}

/**************************************************************
	Problem: 1047
	User: lvrenxiang
	Language: C++
	Result: Wrong Answer
****************************************************************/