#include<bits/stdc++.h>
using namespace std;
int a1,a2,a3,h1,h2,h3,a,h;
int ans;
int main(){
	cin>>a1>>h1>>a2>>h2>>a3>>h3;
	a=a2-a1;
	h=h3-h2;
	ans=a*h/2;
	cout<<ans;
	return 0;
}