#include<bits/stdc++.h>
using namespace std;
int main(){
    int a,b,c;
    float z;
	cin>>a>>b>>c;
	if(a+b>c&&a+c>b&&b+c>a){
		z=(a*b)/2;
		printf("%.1f",z);
	}else{
		cout<<"No";
	}

}

/**************************************************************
	Problem: 1047
	User: zhengzihao3
	Language: C++
	Result: Wrong Answer
****************************************************************/