#include<bits/stdc++.h>
using namespace std;
int main(){
	int a,b,c,s;
	cin>>a>>b>>c;
	if(a*a+b*b==c*c){
		s=a*b/2;
		cout<<s;
	}else cout<<"No";
}
/**************************************************************
	Problem: 1047
	User: toughuatao1
	Language: C++
	Result: Wrong Answer
****************************************************************/