#include <bits/stdc++.h>
using namespace std;
int main(){
   int x,y,z;
   float h;
   cin>>x>>y>>z;
	if (x*x+y*y==z*z){
   h=x*y/2.0;
   printf("%.1f",h);
   } 
   else {
   	cout<<"NO"<<endl;
   }
}
/**************************************************************
	Problem: 1047
	User: xiaojingxuan
	Language: C++
	Result: Wrong Answer
****************************************************************/