#include <bits/stdc++.h>
using namespace std;
int main(){
	float i[3];
	cin>>i[0]>>i[1]>>i[2];
	sort(i,i+3);
	if(i[0]*i[0]+i[1]*i[1]==i[2]*i[2]){
		printf("%.1f",i[0]*i[1]/2);
	}else cout<<"no";
	return 0;
}

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